On the unsteady behaviour of cavity flow over a two-dimensional wall-mounted fence

2019 ◽  
Vol 874 ◽  
pp. 483-525 ◽  
Author(s):  
Luka Barbaca ◽  
Bryce W. Pearce ◽  
Harish Ganesh ◽  
Steven L. Ceccio ◽  
Paul A. Brandner
Keyword(s):  
High Speed ◽  
Single Phase ◽  
Free Stream ◽  
Free Stream Velocity ◽  
Small Scale ◽  
Stream Velocity ◽  
Phase Flow ◽  
Two Dimensional ◽  
Single Phase Flow ◽  
Dynamic Surface

The topology and unsteady behaviour of ventilated and natural cavity flows over a two-dimensional (2-D) wall-mounted fence are investigated for fixed length cavities with varying free-stream velocity using high-speed and still imaging, X-ray densitometry and dynamic surface pressure measurement in two experimental facilities. Cavities in both ventilated and natural flows were found to have a re-entrant jet closure, but not to exhibit large-scale oscillations, yet the irregular small-scale shedding at the cavity closure. Small-scale cavity break-up was associated with a high-frequency broadband peak in the wall pressure spectra, found to be governed by the overlying turbulent boundary layer characteristics, similar to observations from single-phase flow over a forward-facing step. A low-frequency peak reflecting the oscillations in size of the re-entrant jet region, analogous to ‘flapping’ motion in single-phase flow, was found to be modulated by gravity effects (i.e. a Froude number dependence). Likewise, a significant change in cavity behaviour was observed as the flow underwent transition analogous to the transition from sub- to super-critical regime in open-channel flow. Differences in wake topology were examined using shadowgraphy and proper orthogonal decomposition, from which it was found that the size and number of shed structures increased with an increase in free-stream velocity for the ventilated case, while remaining nominally constant in naturally cavitating flow due to condensation of vaporous structures.

Unsteady laminar boundary layers in a compressible stagnation flow

1975 ◽  
Vol 70 (3) ◽  
pp. 561-572 ◽  
Author(s):  
C. S. Vimala ◽  
G. Nath
Keyword(s):  
Free Stream ◽  
Free Stream Velocity ◽  
Stream Velocity ◽  
Compressible Boundary Layer ◽  
Two Dimensional ◽  
Stagnation Flow ◽  
Laminar Boundary Layers ◽  
Implicit Finite Difference ◽  
Layer Flow ◽  
Heat Transfer Parameter

The unsteady laminar compressible boundary-layer flow in the immediate vicinity of a two-dimensional stagnation point due to an incident stream whose velocity varies arbitrarily with time is considered. The governing partial differential equations, involving both time and the independent similarity variable, are transformed into new co-ordinates with finite ranges by means of a transformation which maps an infinite interval into a finite one. The resulting equations are solved by converting them into a matrix equation through the application of implicit finite-difference formulae. Computations have been carried out for two particular unsteady free-stream velocity distributions: (i) a constantly accelerating stream and (ii) a fluctuating stream. The results show that in the former case both the skin-friction and the heat-transfer parameter increase steadily with time after a certain instant, while in the latter they oscillate, thus responding to the fluctuations in the free-stream velocity.

The Effect of Slot Height and Slot-Turbulence Intensity on the Effectiveness of the Uniform Density, Two-Dimensional Wall Jet

1968 ◽  
Vol 90 (4) ◽  
pp. 469-475 ◽  
Author(s):  
S. C. Kacker ◽  
J. H. Whitelaw
Keyword(s):  
Turbulence Intensity ◽  
Free Stream ◽  
Velocity Ratio ◽  
Free Stream Velocity ◽  
Stream Velocity ◽  
Uniform Density ◽  
Wall Jet ◽  
Two Dimensional ◽  
Significant Parameter

This paper presents measurements of the impervious-wall effectiveness of a two-dimensional wall jet obtained using a constant value of lip thickness, 0.032 in. and four slot heights, viz., 0.5, 0.25, 0.132, and 0.074 in. The measurements are in the range of slot-to-free-stream velocity ratio 0.288⩽u¯C/uG⩽2.66 and clearly demonstrate that, in the region where the velocity ratio is close to unity, the effectiveness is greatly dependent upon the slot height. The reason for this dependence is investigated and additional measurements are presented to show that the influence of the slot turbulence intensity on the effectiveness is small. It is concluded that the ratio of slot lip thickness to slot height is the most significant parameter and that the effect of an increase in this ratio is to decrease the effectiveness.

scholarly journals Thermographical study of geometry and phase change influence on PDMS Microchannel liquid cooling devices

2021 ◽  
Vol 2116 (1) ◽  
pp. 012050
Author(s):  
Pedro Pontes ◽  
Lourenço Martins ◽  
Iva Gonçalves ◽  
Ana S Moita ◽  
António LN Moreira
Keyword(s):  
High Speed ◽  
Single Phase ◽  
Phase Flow ◽  
Ir Thermography ◽  
Single Phase Flow ◽  
Liquid Cooling ◽  
Ir Camera ◽  
Pressure Losses ◽  
Cooling Devices ◽  
Pdms Microchannel

Abstract This work proposes a methodology in which high speed camera imaging is combined with infrared (IR) thermography to look at the effect of geometric parameters and boiling in the effectiveness of these coolers. PDMS microchannels were manufactured with 3 channel widths: 250, 500 and 750µm. HFE7100 was used as the refrigerant. Pressure losses were significant for the thinnest geometry as clogging and flow reversal were observed. The dissipated heat flux, as measured by the IR camera was higher in the largest channels, due to the PDMS poor conductivity. Results obtained with HFE7100 were then compared with those obtained with water at single-phase flow. For the same geometry, HFE 7100 resulted in a higher heat transfer coefficient than water.

Origin and Characteristics of the Spray Patterns Generated by Planing Hulls

2011 ◽  
Vol 27 (02) ◽  
pp. 63-83
Author(s):  
Daniel Savitsky ◽  
Michael Morabito†
Keyword(s):  
Free Stream ◽  
Experimental Studies ◽  
Leading Edge ◽  
Free Stream Velocity ◽  
Stream Velocity ◽  
Maximum Height ◽  
Two Dimensional ◽  
Swept Wing ◽  
Analytical Treatment ◽  
Stagnation Line

Analytical and experimental studies were made of the origin and characteristics of the two distinctly different appearing spray patterns associated with prismatic planing hulls. It was found that these two spray patterns, identified as "whisker spray" and "main spray," are a consequence of the same basic hydrodynamic flow phenomenon and transform seamlessly from one into the other. Similar to the analytical treatment developed for swept-wing aircraft, where the oncoming free stream velocity is divided into components normal to and along the leading edge of the swept wing, the free stream velocity for the planing surface is divided into components normal to and along the stagnation line. Combining this orientation of velocity components with the results of analytical studies of Wagner (Uber stoss-und gleitoorgauge an der oberflache von flussiglaseiten. Z.f.a.M.M., 2, 4, 1932) for two-dimensional planing of infinitely long flat planing surfaces with those of Green (The gliding of a flat plate on a stream of finite depth, Proceedings of the Cambridge Philosophical Society, 31, 1935) for planing of a two-dimensional, finite length surface, explains the origin and characteristics of both the whisker and main spray patterns. It is shown that the main spray originates in a small local area at the intersection of the stagnation line with the chine. Relatively simplistic equations that define the three-dimensional location of the spray apex are developed, and the results presented in three graphs. An illustrative example is presented that demonstrates the ease of application of the method to define the main spray geometry of a typical planing craft. Model tests were conducted to define the maximum height of the main spray and its lateral and longitudinal positions relative to the hull as a function of deadrise angle, trim angle, and speed coefficient. These data substantiate the analytical results.

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