scholarly journals Investigation on the wake flow instability behind isolated roughness elements on the forebody of a blunt generic reentry capsule

2017 ◽  
Author(s):  
A. Theiss ◽  
S. Hein
Keyword(s):  
Flow Instability ◽  
Wake Flow ◽  
Roughness Elements

scholarly journals Transition in an infinite swept-wing boundary layer subject to surface roughness and free-stream turbulence

2021 ◽  
Vol 931 ◽  
Author(s):  
Luca De Vincentiis ◽  
Dan S. Henningson ◽  
Ardeshir Hanifi
Keyword(s):  
Boundary Layer ◽  
Free Stream ◽  
Flow Instability ◽  
Transition To Turbulence ◽  
Computational Domain ◽  
Swept Wing ◽  
Free Stream Turbulence ◽  
Roughness Elements ◽  
Layer Flow ◽  
Institute Of Technology

The instability of an incompressible boundary-layer flow over an infinite swept wing in the presence of disc-type roughness elements and free-stream turbulence (FST) has been investigated by means of direct numerical simulations. Our study corresponds to the experiments by Örlü et al. (Tech. Rep., KTH Royal Institute of Technology, 2021, http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-291874). Here, different dimensions of the roughness elements and levels of FST have been considered. The aim of the present work is to investigate the experimentally observed sensitivity of the transition to the FST intensity. In the absence of FST, flow behind the roughness elements with a height above a certain value immediately undergoes transition to turbulence. Impulse–response analyses of the steady flow have been performed to identify the mechanism behind the observed flow instability. For subcritical roughness, the generated wave packet experiences a weak transient growth behind the roughness and then its amplitude decays as it is advected out of the computational domain. In the supercritical case, in which the flow transitions to turbulence, flow as expected exhibits an absolute instability. The presence of FST is found to have a significant impact on the transition behind the roughness, in particular in the case of a subcritical roughness height. For a height corresponding to a roughness Reynolds number $Re_{hh}=461$ , in the absence of FST the flow reaches a steady laminar state, while a very low FST intensity of $Tu =0.03\,\%$ causes the appearance of turbulence spots in the wake of the roughness. These randomly generated spots are advected out of the computational domain. For a higher FST level of $Tu=0.3\,\%$ , a turbulent wake is clearly visible behind the element, similar to that for the globally unstable case. The presented results confirm the experimental observations and explain the mechanisms behind the observed laminar–turbulent transition and its sensitivity to FST.

Study on the Influence of Centrifugal Pump Inlet Flow Field Instability and Methods of Adjustment

2019 ◽  
Author(s):  
Bo Qian ◽  
Jinping Chen ◽  
Peng Wu ◽  
Bin Huang ◽  
Dazhuan Wu
Keyword(s):  
Flow Field ◽  
Centrifugal Pump ◽  
Pressure Fluctuation ◽  
Flow Instability ◽  
Wake Flow ◽  
Experimental Investigations ◽  
Inlet Flow ◽  
Inlet Tube ◽  
Fluctuation Amplitude ◽  
Pump Inlet

Abstract The quality of centrifugal pump inlet flow field is an important factor that affecting the performance of pump. Studies have proved that the vortex in the inlet area can also bring an extra flow instability to pump, which results in a hydro-induced vibration. This phenomenon is more common and significant in the pumps with an elbow tube, which is often applied to minimize installation size, before pump inlet. Therefore, it is necessary to look into the influence of inlet flow field instability on pump performance especially the vibration performance. The methods of adjusting pump inlet flow field is also worthy of being studied in the meantime. In this study, the influence of inlet vortex on the performance of centrifugal pump with an elbow inlet tube is investigated by means of CFD analysis. The flow is significantly affected when going through the elbow tube inlet and then turbulence is generated as a result, which enters the impeller at the next moment. The turbulence brings an asymmetrical flow condition at the impeller suction area, which can intensify pressure pulsation and hydro-induced vibration. In order to reduce the turbulence, two modifications on the elbow inlet tube are investigated in this study. A specially designed vane is deployed inside the inlet tube in the MOD1, and the MOD2 is added with two splitter vanes on the basis of the MOD1. The turbulent flow in the elbow inlet tube can be reordered as it is controlled by the vanes. The difference on pump performances that the inlet vane has made is specifically simulated and compared. The flow fields of the inlet tube influenced by the vane is also investigated on the vortex distribution and velocity vector distribution. The MOD1 has a generally smaller pressure fluctuation amplitude than the prototype in the impeller inlet area while the pressure fluctuation amplitude of the MOD2 in the impeller inlet area is stronger than the prototype. It is considered as a consequence of multiple effects, which are, the rise of velocity because of flow area replacement by the vanes making the flow field less stable as well as the wake flow induced by the vanes increasing the instability of the flow field. Therefore, although the flat vanes can help adjusting the flow field, their negative influences also act in the opposite way. It is worthwhile to find the balance between the benefits and the costs in flow field stability of installing adjusting vanes. The selection of parameter, number and installation position needs to be further investigated. The numerical results of the MOD1 are also validated through experimental investigations.

scholarly journals Pressure gradient effects on wake-flow instabilities behind isolated roughness elements on re-entry capsules

2019 ◽  
Vol 234 (1) ◽  
pp. 28-41
Author(s):  
Alexander Theiss ◽  
Sascha Leyh ◽  
Stefan Hein
Keyword(s):  
Pressure Gradient ◽  
Roughness Element ◽  
Flow Region ◽  
Adverse Pressure Gradient ◽  
Wake Flow ◽  
Heat Shield ◽  
Turbulent Transition ◽  
Hypersonic Wind Tunnel ◽  
Roughness Elements ◽  
Gradient Effects

Laminar-turbulent transition caused by modal disturbance growth in the wake flow of isolated roughness elements on blunt re-entry capsules is studied numerically at typical cold hypersonic wind-tunnel conditions. Two fundamentally different heat shield shapes are considered. On the sphere-cone forebody the wake flow of the roughness is exposed to an adverse pressure gradient, whereas the spherical heat shield exhibits a strongly favorable pressure gradient. The pressure gradient effects on the development of the stationary wake flow and its modal instability characteristics are discussed for various heights and diameters of the cylindrical roughness element. Regions of increased shear develop in its wake, which persist longer in the adverse pressure gradient case. Consequently, the results of spatial two-dimensional eigenvalue analyses reveal that the unstable wake-flow region extends much further downstream and the wake-mode instabilities are considerably more amplified. The disturbance kinetic energy production terms are used to assess the contributions of the different shear-layer regions to the mode growth and its dependence on the pressure gradient.

scholarly journals Nonlinear wakes behind a row of elongated roughness elements

2016 ◽  
Vol 796 ◽  
pp. 516-557 ◽  
Author(s):  
M. E. Goldstein ◽  
Adrian Sescu ◽  
Peter W. Duck ◽  
Meelan Choudhari
Keyword(s):  
Leading Edge ◽  
Wake Flow ◽  
Nonlinear Behaviour ◽  
Wake Region ◽  
Local Boundary ◽  
Reynolds Number Flow ◽  
Downstream Distance ◽  
Roughness Elements ◽  
High Reynolds ◽  
Downstream Flow

This paper is concerned with the high Reynolds number flow over a spanwise-periodic array of roughness elements with interelement spacing of the order of the local boundary-layer thickness. While earlier work by Goldstein et al. (J. Fluid Mech., vol. 644, 2010, pp. 123–163) and Goldstein et al. (J. Fluid Mech., vol. 668, 2011, pp. 236–266) was mainly concerned with smaller roughness heights that produced relatively weak distortions of the downstream flow, the focus here is on extending the analysis to larger roughness heights and streamwise elongated planform shapes that together produce a qualitatively different, nonlinear behaviour of the downstream wakes. The roughness scale flow now has a novel triple-deck structure that is somewhat different from related studies that have previously appeared in the literature. The resulting flow is formally nonlinear in the intermediate wake region, where the streamwise distance is large compared to the roughness dimensions but small compared to the downstream distance from the leading edge, as well as in the far wake region where the streamwise length scale is of the order of the downstream distance from the leading edge. In contrast, the flow perturbations in both of these wake regions were strictly linear in the earlier work by Goldstein et al. (2010, 2011). This is an important difference because the nonlinear wake flow in the present case provides an appropriate basic state for studying the secondary instability and eventual breakdown into turbulence.

scholarly journals The Affect of Wake Flow Instability on Flow Past an Offset-fin Array

1990 ◽  
Vol 10 (1Supplement) ◽  
pp. 27-30
Author(s):  
G.N. XI ◽  
S. FUTAGAMI ◽  
Y. HAGIWARA ◽  
K. SUZUKI
Keyword(s):  
Flow Instability ◽  
Wake Flow ◽  
Flow Past

scholarly journals On the unsteady dynamics of synthetic leaves: Laboratory experiments using synchronized PIV and DIC

2021 ◽  
Vol 1 (1) ◽  
Author(s):  
Lai Wing ◽  
Dan Troolin ◽  
Shyuan Cheng ◽  
Jiao Sun ◽  
Leonardo Chamorro
Keyword(s):  
Flow Instability ◽  
Frame Rate ◽  
Wake Flow ◽  
Image Correlation ◽  
Temperate Climate ◽  
Flow Response ◽  
Image Velocimetry ◽  
Cauchy Numbers ◽  
Windy Weather ◽  
Cauchy Number

The unsteady 3D dynamics of various synthetic leaves and the induced turbulence are systematically studied experimentally for representative Cauchy numbers in a wind tunnel under nearly uniform incoming flows. Synchronized digital image correlation (DIC) and high-frame-rate particle image velocimetry (PIV) are employed to track the structure dynamics simultaneously and the surrounding flow field to uncover the fluid-solid interaction. A high-resolution six-axis load cell is also used to quantify the synthetic leaves' induced force and torque under various flows. The shapes of synthetic leaves inspected are representative of selected environments (e.g., calm to windy weather; tropical to temperate climate). The Cauchy number is set to resemble those observed in natural conditions. This presentation will discuss insights from synchronized PIV-DIC techniques on the synthetic leaves' distinct behavior and wake flow response. Particular emphasis is placed on characterizing flow instability and the leave shape's role in the motions and force. For this purpose, we inspected the instantaneous force and torque as well as their structure. We will also discuss the relationship between leave shapes with force and torque fluctuations linking them with the leaf motion obtained from DIC measurements. In particular, the results show that selected leaf shapes experience significantly larger and distinct force and torque fluctuations and larger pitch magnitude, as shown in Fig. 5. A shared monotonically decreasing trend of the nondimensional frequency (Strouhal number, St = fL/U) is evidenced for standard environmental conditions.

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