scholarly journals Profiles of second- to fourth-order moments of turbulent temperature fluctuations in the convective boundary layer: first measurements with rotational Raman lidar

2015 ◽  
Vol 15 (10) ◽  
pp. 5485-5500 ◽  
Author(s):  
A. Behrendt ◽  
V. Wulfmeyer ◽  
E. Hammann ◽  
S. K. Muppa ◽  
S. Pal
Keyword(s):  
Boundary Layer ◽  
Fourth Order ◽  
Shot Noise ◽  
Temperature Fluctuations ◽  
Atmospheric Temperature ◽  
Order Moment ◽  
Statistical Uncertainty ◽  
Temperature Profiles ◽  
Raman Lidar ◽  
Noise Error

Abstract. The rotational Raman lidar (RRL) of the University of Hohenheim (UHOH) measures atmospheric temperature profiles with high resolution (10 s, 109 m). The data contain low-noise errors even in daytime due to the use of strong UV laser light (355 nm, 10 W, 50 Hz) and a very efficient interference-filter-based polychromator. In this paper, the first profiling of the second- to fourth-order moments of turbulent temperature fluctuations is presented. Furthermore, skewness profiles and kurtosis profiles in the convective planetary boundary layer (CBL) including the interfacial layer (IL) are discussed. The results demonstrate that the UHOH RRL resolves the vertical structure of these moments. The data set which is used for this case study was collected in western Germany (50°53'50.56'' N, 6°27'50.39'' E; 110 m a.s.l.) on 24 April 2013 during the Intensive Observations Period (IOP) 6 of the HD(CP)2 (High-Definition Clouds and Precipitation for advancing Climate Prediction) Observational Prototype Experiment (HOPE). We used the data between 11:00 and 12:00 UTC corresponding to 1 h around local noon (the highest position of the Sun was at 11:33 UTC). First, we investigated profiles of the total noise error of the temperature measurements and compared them with estimates of the temperature measurement uncertainty due to shot noise derived with Poisson statistics. The comparison confirms that the major contribution to the total statistical uncertainty of the temperature measurements originates from shot noise. The total statistical uncertainty of a 20 min temperature measurement is lower than 0.1 K up to 1050 m a.g.l. (above ground level) at noontime; even for single 10 s temperature profiles, it is smaller than 1 K up to 1020 m a.g.l. Autocovariance and spectral analyses of the atmospheric temperature fluctuations confirm that a temporal resolution of 10 s was sufficient to resolve the turbulence down to the inertial subrange. This is also indicated by the integral scale of the temperature fluctuations which had a mean value of about 80 s in the CBL with a tendency to decrease to smaller values towards the CBL top. Analyses of profiles of the second-, third-, and fourth-order moments show that all moments had peak values in the IL around the mean top of the CBL which was located at 1230 m a.g.l. The maximum of the variance profile in the IL was 0.39 K2 with 0.07 and 0.11 K2 for the sampling error and noise error, respectively. The third-order moment (TOM) was not significantly different from zero in the CBL but showed a negative peak in the IL with a minimum of −0.93 K3 and values of 0.05 and 0.16 K3 for the sampling and noise errors, respectively. The fourth-order moment (FOM) and kurtosis values throughout the CBL were not significantly different to those of a Gaussian distribution. Both showed also maxima in the IL but these were not statistically significant taking the measurement uncertainties into account. We conclude that these measurements permit the validation of large eddy simulation results and the direct investigation of turbulence parameterizations with respect to temperature.

scholarly journals Profiles of second- to third-order moments of turbulent temperature fluctuations in the convective boundary layer: first measurements with Rotational Raman Lidar

2014 ◽  
Vol 14 (21) ◽  
pp. 29019-29055 ◽  
Author(s):  
A. Behrendt ◽  
V. Wulfmeyer ◽  
E. Hammann ◽  
S. K. Muppa ◽  
S. Pal
Keyword(s):  
Boundary Layer ◽  
Temperature Measurement ◽  
Convective Boundary Layer ◽  
Temperature Fluctuations ◽  
Atmospheric Temperature ◽  
Order Moment ◽  
Temperature Profiles ◽  
Raman Lidar ◽  
Third Order ◽  
Convective Boundary

Abstract. The rotational Raman lidar of the University of Hohenheim (UHOH) measures atmospheric temperature profiles during daytime with high resolution (10 s, 109 m). The data contain low noise errors even in daytime due to the use of strong UV laser light (355 nm, 10 W, 50 Hz) and a very efficient interference-filter-based polychromator. In this paper, we present the first profiling of the second- to forth-order moments of turbulent temperature fluctuations as well as of skewness and kurtosis in the convective boundary layer (CBL) including the interfacial layer (IL). The results demonstrate that the UHOH RRL resolves the vertical structure of these moments. The data set which is used for this case study was collected in western Germany (50°53'50.56′′ N, 6°27'50.39′′ E, 110 m a.s.l.) within one hour around local noon on 24 April 2013 during the Intensive Observations Period (IOP) 6 of the HD(CP)2 Observational Prototype Experiment (HOPE), which is embedded in the German project HD(CP)2 (High-Definition Clouds and Precipitation for advancing Climate Prediction). First, we investigated profiles of the noise variance and compared it with estimates of the statistical temperature measurement uncertainty Δ T based on Poisson statistics. The agreement confirms that photon count numbers obtained from extrapolated analog signal intensities provide a lower estimate of the statistical errors. The total statistical uncertainty of a 20 min temperature measurement is lower than 0.1 K up to 1050 m a.g.l. at noontime; even for single 10 s temperature profiles, it is smaller than 1 K up to 1000 m a.g.l.. Then we confirmed by autocovariance and spectral analyses of the atmospheric temperature fluctuations that a temporal resolution of 10 s was sufficient to resolve the turbulence down to the inertial subrange. This is also indicated by the profile of the integral scale of the temperature fluctuations, which was in the range of 40 to 120 s in the CBL. Analyzing then profiles of the second-, third-, and forth-order moments, we found the largest values of all moments in the IL around the mean top of the CBL which was located at 1230 m a.g.l. The maximum of the variance profile in the IL was 0.40 K2 with 0.06 and 0.08 K2 for the sampling error and noise error, respectively. The third-order moment was not significantly different from zero inside the CBL but showed a negative peak in the IL with a minimum of −0.72 K3 and values of 0.06 and 0.14 K3 for the sampling and noise errors, respectively. The forth-order moment and kurtosis values throughout the CBL were quasi-normal.

scholarly journals Characterization of Boundary Layer Turbulent Processes by the Raman Lidar BASIL in the frame of HD(CP)<sup>2</sup>) Observational Prototype Experiment

2016 ◽  
Author(s):  
Paolo Di Girolamo ◽  
Marco Cacciani ◽  
Donato Summa ◽  
Andrea Scoccione ◽  
Benedetto De Rosa ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Water Vapour ◽  
Temporal Resolution ◽  
Fourth Order ◽  
Temperature Fluctuations ◽  
Temperature Measurements ◽  
Order Moment ◽  
Mixing Ratio ◽  
Raman Lidar ◽  
Turbulent Fluctuations

Abstract. Measurements carried out by the University of BASILicata Raman lidar system (BASIL) are reported to demonstrate the capability of this instrument to characterize turbulent processes within the convective boundary layer (CBL). In order to resolve the vertical profiles of turbulent variables, high resolution water vapour and temperature measurements, with a temporal resolution of 10 s and a vertical resolution of 90 m and 30 m, respectively, are considered. Measurements of higher-order moments of the turbulent fluctuations of water vapour mixing ratio and temperature are obtained based on the application of auto-covariance analyses to the water vapour mixing ratio and temperature time series. The algorithms are applied to a case study (11:30–13:30 UTC, 20 April 2013) from the High Definition Clouds and Precipitation for Climate Prediction (HD(CP)2) Observational Prototype Experiment (HOPE), held in Western Germany in the spring 2013. A new correction scheme for the elastic-signal leakage in the low-quantum number rotational Raman signal is applied. The noise errors are small enough to derive up to fourth-order moments for both water vapour mixing ratio and temperature fluctuations. To the best of our knowledge, BASIL is the first Raman lidar with a demonstrated capability to simultaneously retrieve daytime profiles of water vapour turbulent fluctuations up to the fourth order throughout the atmospheric CBL, this capability being combined with the one to also measure daytime profiles of temperature fluctuations up to the fourth order. For the considered case study, which represents a well-mixed and quasi-stationary CBL, the mean boundary layer height is found to be 1290 ± 77 m a.g.l. Values of the integral scale for water vapour and temperature fluctuations at the top of the CBL are in the range of 70–125 s and 75–225 s, respectively; these values are much larger than the temporal resolution of the measurements (10 s), which testifies that the temporal resolution considered for the measurements is sufficiently high to resolve turbulence processes down to the inertial sub-range and consequently resolve the major part of the turbulent fluctuations. Peak values of all moments are found in the interfacial layer in the proximity of the top of the CBL. Specifically, water vapour and temperature second-order moment (variance) has a maximum value of 0.29 g2 kg−2 and 0.26 K2, respectively, water vapour and temperature third-order moment has a peak value of 0.156 g3 kg−3 and −0.067 K3, respectively, while water vapour and temperature fourth-order moment has a maximum value of 0.28 g4 kg−4 and 0.24 K4, respectively. Water vapour and temperature kurtosis have values of ~ 3 in the entrainment zone, which indicate normally distributed humidity and temperature fluctuations. Reported values of the higher-order moments result to be in good agreement with previous measurements at different locations, thus providing confidence on the possibility to use them for turbulence parameterization in weather and climate models. In the determination of the temperature profiles, particular care was dedicated to minimize potential effects associated with elastic signal leakage in the rotational Raman signals. For this purpose, a specific algorithm was defined and tested to identify and remove signal leakages and to assess the residual systematic uncertainty affecting temperature measurements after correction. The application of this approach confirms that for the present Raman lidar system the leakage factor keeps constant with time, and consequently an appropriate assessment of its constant value allows for a complete removal of the leaking elastic signal from the rotational Raman lidar signals at any time (with a residual error on temperature measurements after correction not exceeding 0.16 K).

scholarly journals Compact Operational Tropospheric Water Vapor and Temperature Raman Lidar  with Turbulence Resolution

2021 ◽  
Author(s):  
Diego Lange ◽  
Andreas Behrendt ◽  
Volker Wulfmeyer
Keyword(s):  
Boundary Layer ◽  
Water Vapor ◽  
Data Assimilation ◽  
Inversion Layer ◽  
Lower Troposphere ◽  
Statistical Uncertainty ◽  
Temperature Profiles ◽  
Free Troposphere ◽  
Earth System ◽  
Raman Lidar

&lt;p&gt;We present the Atmospheric Raman Temperature and Humidity Sounder (ARTHUS), a new tool for observations in the atmospheric boundary layer and lower free troposphere during daytime and nighttime with very high resolution up to the turbulence scale, high accuracy and precision, and very short latency and illustrate its performance with new measurements examples. ARTHUS measurements resolve the strength of the inversion layer at the planetary boundary layer top, elevated lids in the free troposphere, and turbulent fluctuations in water vapor and temperature, simultaneously (Lange et al., 2019). In addition to thermodynamic variables, ARTHUS provides also independent profiles of the particle backscatter coefficient and the particle extinction coefficient from the rotational Raman signals at 355 nm with much better resolution than a conventional vibrational Raman lidar.&lt;/p&gt;&lt;p&gt;The observation of atmospheric moisture and temperature profiles is essential for the understanding and prediction of earth system processes. These are fundamental components of the global and regional energy and water cycles, they determine the radiative transfer through the atmosphere, and are critical for the cloud formation and precipitation (Wulfmeyer, 2015). Also, as confirmed by case studies, the assimilation of high-quality, lower tropospheric WV and T profiles results in a considerable improvement of the skill of weather forecast models particularly with respect to extreme events.&lt;/p&gt;&lt;p&gt;Very stable and reliable performance was demonstrated during more than 3000 hours of operation experiencing a huge variety of weather conditions, including seaborne operation during the EUREC4A campaign (Bony et al., 2017, Stevens et al., 2020). ARTHUS provides temperature profiles with resolutions of 10-60 s and 7.5-100 m vertically in the lower free troposphere. During daytime, the statistical uncertainty of the WV mixing ratio is &lt;2 % in the lower troposphere for resolutions of 5 minutes and 100 m. Temperature statistical uncertainty is &lt;0.5 K even up to the middle troposphere. Consequently, ARTHUS fulfills the stringent WMO breakthrough requirements on nowcasting and very short-range forecasting (see www. wmo&amp;#8208;sat.info/oscar/observingrequirements).&lt;/p&gt;&lt;p&gt;This performance serves very well the next generation of very fast rapid-update-cycle data assimilation systems. Ground-based stations and networks can be set up or extended for climate monitoring, verification of weather, climate and earth system models, data assimilation for improving weather forecasts.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References:&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;Bony et al., 2017, https://doi.org/10.1007/s10712-017-9428-0&lt;/p&gt;&lt;p&gt;Lange et al., 2019, https://doi.org/10.1029/2019GL085774&lt;/p&gt;&lt;p&gt;Stevens et al. 2020, submitted to ESSD&lt;/p&gt;&lt;p&gt;Wulfmeyer et al., 2015, doi:10.1002/2014RG000476&lt;/p&gt;

scholarly journals Characterisation of boundary layer turbulent processes by the Raman lidar BASIL in the frame of HD(CP)<sup>2</sup> Observational Prototype Experiment

2017 ◽  
Vol 17 (1) ◽  
pp. 745-767 ◽  
Author(s):  
Paolo Di Girolamo ◽  
Marco Cacciani ◽  
Donato Summa ◽  
Andrea Scoccione ◽  
Benedetto De Rosa ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Water Vapour ◽  
Temporal Resolution ◽  
Fourth Order ◽  
Temperature Fluctuations ◽  
Temperature Measurements ◽  
Mixing Ratio ◽  
Raman Lidar ◽  
Turbulent Fluctuations ◽  
Signal Crosstalk

Abstract. Measurements carried out by the University of Basilicata Raman lidar system (BASIL) are reported to demonstrate the capability of this instrument to characterise turbulent processes within the convective boundary layer (CBL). In order to resolve the vertical profiles of turbulent variables, high-resolution water vapour and temperature measurements, with a temporal resolution of 10 s and vertical resolutions of 90 and 30 m, respectively, are considered. Measurements of higher-order moments of the turbulent fluctuations of water vapour mixing ratio and temperature are obtained based on the application of autocovariance analyses to the water vapour mixing ratio and temperature time series. The algorithms are applied to a case study (11:30–13:30 UTC, 20 April 2013) from the High Definition Clouds and Precipitation for Climate Prediction (HD(CP)2) Observational Prototype Experiment (HOPE), held in western Germany in the spring 2013. A new correction scheme for the removal of the elastic signal crosstalk into the low quantum number rotational Raman signal is applied. The noise errors are small enough to derive up to fourth-order moments for both water vapour mixing ratio and temperature fluctuations.To the best of our knowledge, BASIL is the first Raman lidar with a demonstrated capability to simultaneously retrieve daytime profiles of water vapour turbulent fluctuations up to the fourth order throughout the atmospheric CBL. This is combined with the capability of measuring daytime profiles of temperature fluctuations up to the fourth order. These measurements, in combination with measurements from other lidar and in situ systems, are important for verifying and possibly improving turbulence and convection parameterisation in weather and climate models at different scales down to the grey zone (grid increment  ∼  1 km; Wulfmeyer et al., 2016).For the considered case study, which represents a well-mixed and quasi-stationary CBL, the mean boundary layer height is found to be 1290 ± 75 m above ground level (a.g.l.). Values of the integral scale for water vapour and temperature fluctuations at the top of the CBL are in the range of 70–125 and 75–225 s, respectively; these values are much larger than the temporal resolution of the measurements (10 s), which testifies that the temporal resolution considered for the measurements is sufficiently high to resolve turbulent processes down to the inertial subrange and, consequently, to resolve the major part of the turbulent fluctuations. Peak values of all moments are found in the interfacial layer in the proximity of the top of the CBL. Specifically, water vapour and temperature second-order moments (variance) have maximum values of 0.29 g2 kg−2 and 0.26 K2; water vapour and temperature third-order moments have peak values of 0.156 g3 kg−3 and −0.067 K3, while water vapour and temperature fourth-order moments have maximum values of 0.28 g4 kg−4 and 0.24 K4. Water vapour and temperature kurtosis have values of  ∼  3 in the upper portion of the CBL, which indicate normally distributed humidity and temperature fluctuations. Reported values of the higher-order moments are in good agreement with previous measurements at different locations, thus providing confidence in the possibility of using these measurements for turbulence parameterisation in weather and climate models.In the determination of the temperature profiles, particular care was dedicated to minimise potential effects associated with elastic signal crosstalk on the rotational Raman signals. For this purpose, a specific algorithm was defined and tested to identify and remove the elastic signal crosstalk and to assess the residual systematic uncertainty affecting temperature measurements after correction. The application of this approach confirms that, for the present Raman lidar system, the crosstalk factor remains constant with time; consequently an appropriate assessment of its constant value allows for a complete removal of the leaking elastic signal from the rotational Raman lidar signals at any time (with a residual error on temperature measurements after correction not exceeding 0.18 K).

Measurement of Atmospheric Temperature Profiles Using Raman Lidar

1979 ◽  
Vol 18 (2) ◽  
pp. 225-227 ◽  
Author(s):  
R. Gill ◽  
K. Geller ◽  
J. Farina ◽  
J. Cooney ◽  
A. Cohen
Keyword(s):  
Atmospheric Temperature ◽  
Temperature Profiles ◽  
Raman Lidar

scholarly journals Operational profiling of temperature using ground-based microwave radiometry at Payerne: prospects and challenges

2012 ◽  
Vol 5 (5) ◽  
pp. 1121-1134 ◽  
Author(s):  
U. Löhnert ◽  
O. Maier
Keyword(s):  
Boundary Layer ◽  
Data Quality ◽  
Microwave Radiometer ◽  
Lower Boundary ◽  
Weather Conditions ◽  
Atmospheric Temperature ◽  
Temperature Profiles ◽  
Data Quality Control ◽  
Time Period ◽  
Lower Boundary Layer

Abstract. The motivation of this study is to verify theoretical expectations placed on ground-based microwave radiometer (MWR) techniques and to confirm whether they are suitable for supporting key missions of national weather services, such as timely and accurate weather advisories and warnings. We evaluate reliability and accuracy of atmospheric temperature profiles retrieved continuously by the microwave profiler system HATPRO (Humidity And Temperature PROfiler) operated at the aerological station of Payerne (MeteoSwiss) in the time period August 2006–December 2009. Assessment is performed by comparing temperatures from the radiometer against temperature measurements from a radiosonde accounting for a total of 2107 quality-controlled all-season cases. In the evaluated time period, the MWR delivered reliable temperature profiles in 86% of all-weather conditions on a temporal resolution of 12–13 min. Random differences between MWR and radiosonde are down to 0.5 K in the lower boundary layer and increase to 1.7 K at 4 km height. The differences observed between MWR and radiosonde in the lower boundary layer are similar to the differences observed between the radiosonde and another in-situ sensor located on a close-by 30 m tower. Temperature retrievals from above 4 km contain less than 5% of the total information content of the measurements, which makes clear that this technique is mainly suited for continuous observations in the boundary layer. Systematic temperature differences are also observed throughout the retrieved profile and can account for up to ±0.5 K. These errors are due to offsets in the measurements of the microwave radiances that have been corrected for in data post-processing and lead to nearly bias-free overall temperature retrievals. Different reasons for the radiance offsets are discussed, but cannot be unambiguously determined retrospectively. Monitoring and, if necessary, corrections for radiance offsets as well as a real-time rigorous automated data quality control are mandatory for microwave profiler systems that are designated for operational temperature profiling. In the analysis of a subset of different atmospheric situations, it is shown that lifted inversions and data quality during precipitation present the largest challenges for operational MWR temperature profiling.

scholarly journals Assessing the potential of passive microwave radiometers for continuous temperature profile retrieval using a three year data set from Payerne

2011 ◽  
Vol 4 (6) ◽  
pp. 7435-7469 ◽  
Author(s):  
U. Löhnert ◽  
O. Maier
Keyword(s):  
Boundary Layer ◽  
Lower Boundary ◽  
Weather Conditions ◽  
Atmospheric Temperature ◽  
Temperature Measurements ◽  
Temperature Profiles ◽  
Data Quality Control ◽  
Data Set ◽  
Time Period ◽  
Lower Boundary Layer

Abstract. The motivation of this study is to verify theoretical expectations placed on ground-based radiometer techniques and to confirm whether they are suitable for supporting key missions of national weather services, such as timely and accurate weather advisories and warnings. We evaluate reliability and accuracy of atmospheric temperature profiles retrieved continuously by a HATPRO (Humidity And Temperature PROfiler) system operated at the aerological station of Payerne (MeteoSwiss) in the time period August 2006–December 2009. Assessment is performed by comparing temperatures from the radiometer against temperature measurements from a radiosonde accounting for a total of 2088 quality-controlled all-season cases. In the evaluated time period, HATPRO delivered reliable temperature profiles in 88% of all-weather conditions with a temporal resolution of 15 min. Random differences between HATPRO and radiosonde are down to 0.5 K in the lower boundary layer and rise up to 1.7 K at 4 km height. The differences observed between HATPRO and radiosonde in the lower boundary layer are similar to the differences observed between the radiosonde and another in-situ sensor located on a close-by 30 m tower. Temperature retrievals from above 4 km contain less than 5% of the total information content of the measurements, which makes clear that this technique is mainly suited for continuous observations in the boundary layer. Systematic temperature differences are also observed throughout the retrieved profile and can account for up to ±0.5 K. These errors are due to offsets in the measurements of the microwave radiances that have been corrected for in data post-processing and lead to nearly bias-free overall temperature retrievals. Different reasons for the radiance offsets are discussed, but cannot be unambiguously determined retrospectively. Monitoring and, if necessary, corrections for radiance offsets as well as a real-time rigorous automated data quality control are mandatory for microwave profiler systems that are designated for operational temperature profiling. In the analysis of day/night differences, it is shown that systematic differences between radiosonde and HATPRO decrease throughout the boundary layer if 2 m surface temperature measurements are included in the retrieval.

scholarly journals Temperature profiles with bi-static Doppler-RASS and their accuracy

2012 ◽  
Vol 5 (1) ◽  
pp. 1075-1100
Author(s):  
B. Hennemuth ◽  
G. Peters ◽  
H.-J. Kirtzel
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Atmospheric Temperature ◽  
Measuring Technique ◽  
Temperature Profiles ◽  
Antenna Aperture ◽  
Scattering Angle ◽  
Retrieval Scheme ◽  
Set Up

Abstract. The technique of atmospheric temperature profiling by Doppler-RASS is discussed. The set up with bi-static (separated transmit and receiving) antennas implies a range dependent scattering angle. The retrieval scheme developed by Kon for such antenna geometry is reviewed and its limits of validity are discussed. Empirical tuning of the efficient antenna aperture is proposed to fit the retrieved temperature profiles to reality. Examples of application of the measuring technique for atmospheric boundary layer characterization are presented.

Flow and Heat Transfer Behavior in Transitional Boundary Layers With Streamwise Acceleration

1996 ◽  
Vol 118 (2) ◽  
pp. 314-326 ◽  
Author(s):  
F. J. Keller ◽  
T. Wang
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Reynolds Number ◽  
Pressure Gradient ◽  
Temperature Fluctuations ◽  
Heat Fluxes ◽  
Temperature Profiles ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Free Stream Turbulence

The effects of streamwise acceleration on a two-dimensional heated boundary layer undergoing natural laminar-turbulent transition were investigated with detailed measurements of momentum and thermal transport phenomena. Tests were conducted over a heated flat wall with zero pressure-gradient and three levels of streamwise acceleration: K ≡ (v/U∞2) (d/U∞/dx) = 0.07, 0.16, and 0.25 × 10−6. Free-stream turbulence intensities were maintained at approximately 0.5 percent for the baseline case and 0.4 percent for the accelerating cases. A miniature three-wire probe was used to measure mean velocity and temperature profiles, Reynolds stresses, and Reynolds heat fluxes. Transition onset and end were inferred from Stanton numbers and skin-friction coefficients. The results indicate that mild acceleration delays transition onset and increases transition length both in terms of distance, x, and Reynolds number based on x. Transition onset and length are relatively insensitive to acceleration in terms of momentum thickness Reynolds number. This is supported by the boundary layer thickness and integral parameters, which indicate that a favorable pressure gradient suppresses boundary layer growth and development in the transition region. Heat transfer rates and temperature profiles in the late-transition and early-turbulent regions lag behind the development of wall shear stress and velocity profiles. This lag increases as K increases, indicating that the evolution of the heat transport is slower than that of the momentum transport. Comparison of the evolution of rms temperature fluctuations to the evolution of Reynolds normal stresses indicates a similar lag in the rms temperature fluctuations.

Flow and Heat Transfer Behavior in Transitional Boundary Layers With Streamwise Acceleration

1994 ◽  
Author(s):  
F. Jeffrey Keller ◽  
Ting Wang
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Reynolds Number ◽  
Pressure Gradient ◽  
Temperature Fluctuations ◽  
Heat Fluxes ◽  
Temperature Profiles ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Free Stream Turbulence

The effects of streamwise acceleration on a two-dimensional heated boundary layer undergoing natural laminar-turbulent transition were investigated with detailed measurements of momentum and thermal transport phenomena. Tests were conducted over a heated flat wall with zero pressure-gradient and three levels of streamwise acceleration: K≡νU¯∞2dU¯∞dx= 0.07, 0.16, and 0.25 × 10−6. Free-stream turbulence intensities were maintained at approximately 0.5% for the baseline case and 0.4% for the accelerating cases. A miniature three-wire probe was used to measure mean velocity and temperature profiles, Reynolds stresses, and Reynolds heat fluxes. Transition onset and end were inferred from Stanton numbers and skin-friction coefficients. The results indicate that mild acceleration delays transition onset and increases transition length both in terms of distance, x1 and Reynolds number based on x. Transition onset and length are relatively insensitive to acceleration in terms of momentum thickness Reynolds number. This is supported by the boundary layer thickness and integral parameters which indicate that a favorable pressure gradient suppresses boundary layer growth and development in the transition region. Heat transfer rates and temperature profiles in the late-transition and early-turbulent regions lag behind the development of wall shear stress and velocity profiles. This lag increases as K increases, indicating that the evolution of the heat transport is slower than that of the momentum transport. Comparison of the evolution of RMS temperature fluctuations to the evolution of Reynolds normal stresses indicates a similar lag in the RMS temperature fluctuations.

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