Symmetric Holmboe instabilities in a laboratory exchange flow

2009 ◽  
Vol 636 ◽  
pp. 137-153 ◽  
Author(s):  
EDMUND W. TEDFORD ◽  
R. PIETERS ◽  
G. A. LAWRENCE
Keyword(s):  
Cross Section ◽  
Laboratory Experiments ◽  
Rectangular Cross Section ◽  
Flow Direction ◽  
Phase Speed ◽  
Exchange Flow ◽  
Subcritical Region ◽  
Average Wavelength

Laboratory experiments have been conducted that test the predictions of Holmboe (Geofys. Publ., vol. 24, 1962, pp. 67–112). Symmetric Holmboe instabilities are observed during steady, maximal two-layer exchange flow in a long laboratory channel of rectangular cross-section. Internal hydraulic controls at each end of the channel isolate the subcritical region within the channel from disturbances in the reservoirs. Inside the channel, the instabilities form cusp-like waves that propagate in both directions. The phase speed of the instabilities is consistent with Holmboe's theory and increases along the length of the channel as a result of the gradual acceleration of each layer. This acceleration causes the wavelength of any given instability to increase in the flow direction until it is approximately twice the most amplified wavelength. At this point new waves develop with the result that the average wavelength is almost constant along the length of the channel.

Comparative Study of Thermal Performance of Longitudinal and Transversal-Wavy Microchannel Heat Sinks for Electronic Cooling

2013 ◽  
Vol 135 (2) ◽  
Author(s):  
Gongnan Xie ◽  
Jian Liu ◽  
Yanquan Liu ◽  
Bengt Sunden ◽  
Weihong Zhang
Keyword(s):  
Reynolds Number ◽  
Pressure Drop ◽  
Thermal Resistance ◽  
Cross Section ◽  
Thermal Performance ◽  
Heat Load ◽  
Rectangular Cross Section ◽  
Flow Direction ◽  
Heat Sinks ◽  
Liquid Cooling

Liquid cooling incorporating microchannels are used to cool electronic chips in order to remove more heat load. However, such microchannels are often designed to be straight with rectangular cross section. In this paper, on the basis of straight microchannels having rectangular cross section (SRC), longitudinal-wavy microchannel (LWC), and transversal microchannel (TWC) were designed, respectively, and then the corresponding laminar flow and heat transfer were investigated numerically. Among them, the channel wall of LWC undulates along the flow direction according to a sinusoidal function while the TWC undulates along the transversal direction. The numerical results show that for removing an identical heat load, the overall thermal resistance of the LWC is decreased with increasing inlet Reynolds number while the pressure drop is increased greatly, so that the overall thermal performance of LWC is inferior to that of SRC under the considered geometries. On the contrary, TWC has a great potential to reduce the pressure drop compared to SRC, especially for higher wave amplitudes at the same Reynolds number. Thus the overall thermal performance of TWC is superior to that of SRC. It is suggested that the TWC can be used to cool chips effectively with much smaller pressure drop penalty. In addition to the overall thermal resistance, other criteria of evaluation of the overall thermal performance, e.g., (Nu/Nu0)/(f/f0) and (Nu/Nu0)/(f/f0)1/3, are applied and some controversial results are obtained.

Experimental Study of Heat Transfer Characteristics of a Bluff Body Interacting With a Rib by Using Liquid Crystal Thermography

2013 ◽  
Author(s):  
Zahra Ghorbani-Tari ◽  
Lei Wang ◽  
Bengt Sunden
Keyword(s):  
Heat Transfer ◽  
Liquid Crystal ◽  
Cross Section ◽  
Bluff Body ◽  
Rectangular Cross Section ◽  
Flow Direction ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Upstream Region ◽  
Liquid Crystal Thermography

In the present study, the effects of a single rib on the local heat transfer around a bluff body are examined by using the steady state liquid crystal technique. By varying the spacing between the bluff body and the rib, the interaction between the rib and the bluff body can be controlled. The bluff body is oriented vertically towards the flow direction and spans the whole height of the channel. The bluff body has a rectangular cross-section (height b = 80 mm, width 40 mm) while the rib has a square cross-section (rib height e = 10 mm, rib width 10 mm). The rib is placed in the upstream region of the bluff body either by a spacing L = 100 and 50 mm to yield non-dimensional spacing to rib height ratios L/e = 10 and 5 respectively. Here, the re-attachment length (xR) for a single rib is about 7.5e. The values of the Reynolds number based on the channel hydraulic diameter (Dh) are 55,000 and 89,000. The experimental results revealed that the presence of the rib at L/e = 10 has a more pronounced effect on the enhancement of the heat transfer upstream region of the bluff body. The effect of different L/e on the local Nu number distribution in the wake of the bluff body is small.

Multi-layer hydraulic exchange flows

2000 ◽  
Vol 416 ◽  
pp. 269-296 ◽  
Author(s):  
G. F. LANE-SERFF ◽  
D. A. SMEED ◽  
C. R. POSTLETHWAITE
Keyword(s):  
Internal Wave ◽  
Laboratory Experiments ◽  
Rectangular Cross Section ◽  
Wave Mode ◽  
Uniform Density ◽  
Exchange Flow ◽  
Hydraulic Control ◽  
Narrow Channels ◽  
Reservoir Conditions ◽  
Flow Through

Flows between ocean basins are often controlled by narrow channels and shallow sills. A multi-layer hydraulic control theory is developed for exchange flow through such constrictions. The theory is based on the inviscid shallow-water equations and extends the functional approach introduced by Gill (1977) and developed by Dalziel (1991). The flows considered are those in rectangular–cross-section channels connecting two large reservoirs, with a single constriction (sill and/or narrows). The exchange flow depends on the stratification in the two reservoirs, represented as a finite number of immiscible layers of (different) uniform density. For most cases the flow is ‘controlled’ at the constriction and often at other points along the channel (virtual controls) too. As with one- and two-layer hydraulics, controls are locations at which the flow passes from one solution branch to another, and at which (at least) one internal wave mode is stationary. The theory is applied to three-layer flows, which have two internal wave modes, predicting interface heights and layer fluxes from the given reservoir conditions. The theoretical results for three-layer flows are compared to a comprehensive set of laboratory experiments and found to give good agreement. The laboratory experiments also show other features of the flow, such as the formation of waves on the interfaces. The implications of the results for oceanographic flows and ocean modelling are discussed.

Lock-Exchange Flows in Non-Rectangular Cross-Section Channels

2004 ◽  
Vol 126 (2) ◽  
pp. 290-292 ◽  
Author(s):  
Luis Thomas and ◽  
Beatriz Marino
Keyword(s):  
Analytical Model ◽  
Cross Section ◽  
Laboratory Experiments ◽  
Rectangular Cross Section ◽  
Hydraulic Engineering ◽  
Stratified Flows ◽  
Straight Channel ◽  
Simple Analytical Model ◽  
Engineering Structures ◽  
Exchange Flows

Lock-exchange flows driven by density differences in non-rectangular cross-section channels are investigated in situations that resemble estuaries, navigation canals and hydraulic engineering structures. A simple analytical model considering stratified flows suggests practical relationships corroborated by results of laboratory experiments carried out in a straight channel of triangular cross-section.

Front Condition for Gravity Currents in Channels of Nonrectangular Symmetric Cross-Section Shapes

2009 ◽  
Vol 131 (5) ◽  
Author(s):  
B. M. Marino ◽  
L. P. Thomas
Keyword(s):  
Froude Number ◽  
Cross Section ◽  
Laboratory Experiments ◽  
Rectangular Cross Section ◽  
Theoretical Models ◽  
Gravity Currents ◽  
Cross Section Shape ◽  
Section Shape ◽  
Front Condition ◽  
Upper Boundary

We study the variation of the Froude number at the front of gravity currents developed in uniform channels whose cross-section shape depends on a parameter usually used in many numerical and theoretical models. The thickness and front velocity of the dense currents running on the bottom are greater for all the cases studied, resulting in a Froude number greater than that corresponding to the rectangular cross-section shape. The light currents developing along the upper boundary show the opposite trend. It is found that the results are not related to the depth and width of the channel. The relationships obtained agree with the results of laboratory experiments in which open and closed channels of different cross-section shapes are used.

scholarly journals AN EXPERIMENTAL INVESTIGATION OF STATIC PRESSURE DISTRIBUTIONS ON A GROUP OF SQUARE OR RECTANGULAR CYLINDERS WITH ROUNDED CORNERS

1970 ◽  
Vol 41 (1) ◽  
pp. 42-49 ◽  
Author(s):  
A.C. Mandal ◽  
G. M. G. Faruk
Keyword(s):  
Cross Section ◽  
Static Pressure ◽  
Rectangular Cross Section ◽  
Flow Direction ◽  
Tall Buildings ◽  
Wind Load ◽  
Open Circuit ◽  
The Other ◽  
Appreciable Effect ◽  
Pressure Distributions

The static pressure distributions on a group of cylinders with either square or rectangular cross-section having rounded corners are presented. The test is conducted at the exit end of an open circuit wind tunnel with uniform velocity of 14.2 m/s across the cylinder with Reynolds number of 2.87 x 104 and 4.20 x 104 based on the side dimension of the cylinder normal to the direction of the approach flow. The experiment is performed for a group consisting of two cylinders one behind the other along the flow direction with different side dimensions at zero angle of attack for various interspacing between the cylinders. It is observed from the experimental results that there is appreciable effect of the side dimension and interspacing on the drag coefficient of the cylinders. The results are applicable to a group consisting of two tall buildings one behind the other along the wind velocity direction and each building of either square or rectangular cross-section having rounded corners. The wind load on the downstream cylinder decreases remarkably due to the presence of the cylinder in the upstream side.Keywords: Wind load; Group of buildings.DOI: 10.3329/jme.v41i1.5361Journal of Mechanical Engineering, Vol. ME 41, No. 1, June 2010 42-49

The effect of aspect ratio on longitudinal diffusivity in rectangular channels

1982 ◽  
Vol 120 ◽  
pp. 347-358 ◽  
Author(s):  
P. C. Chatwin ◽  
Paul J. Sullivan
Keyword(s):  
Laminar Flow ◽  
Aspect Ratio ◽  
Cross Section ◽  
Laboratory Experiments ◽  
Rectangular Cross Section ◽  
Independent Method ◽  
Surprising Result ◽  
Large Aspect Ratio ◽  
Rectangular Channels ◽  
Side Walls

In a recent paper Doshi, Daiya & Gill (1978) showed that the value of Taylor's longitudinal diffusivity D for laminar flow in a channel of rectangular cross-section of breadth u and height b is about 8D0, for large values of the aspect ratio a/b, where Do is the value of the longitudinal diffusivity obtained by ignoring all variation across the channel. This superficially surprising result is confirmed by an independent method, and is shown to be caused by the boundary layers on the side walls of the channel. The primary purpose of the paper, however, is to consider the value of D in turbulent flow in a flat-bottomed channel of large aspect ratio, for which arguments based on physics are adduced in support of the formula D≈[1 + B][1 - λ(b/u)], where B and λ are positive constants independent of b. It is shown that this result is consistent with laboratory experiments by Fischer (1966). The paper concludes with a discussion of the practical effects of aspect ratio on longitudinal dispersion in channels whose cross-section is approximately rectangular.

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