Influence of Viscous Effects on Impact Tubes

1953 ◽  
Vol 20 (2) ◽  
pp. 253-256
Author(s):  
C. W. Hurd ◽  
K. P. Chesky ◽  
A. H. Shapiro
Keyword(s):  
Reynolds Number ◽  
Free Stream ◽  
Pressure Coefficient ◽  
Pressure Rise ◽  
Experimental Investigations ◽  
Stream Velocity ◽  
Fluid Density ◽  
Viscous Effects ◽  
Point Pressure ◽  
The Impact

Abstract Experiments were conducted to determine the effect of viscosity on the pressure rise recorded by a blunt-nosed impact tube in incompressible flow. The results are presented in terms of the pressure coefficient (Cp ≡ 2Δp/ρV∞2) as a function of Reynolds number (Rey ≡ V∞α/ν), where Δp is the excess of stagnation-point pressure over free-stream static pressure, V∞ is the free-stream velocity, α is the radius of the impact tube, ρ is the fluid density, and ν is the kinematic viscosity of the fluid. Above Reynolds numbers of 1000, there is no effect of viscosity, and Cp is equal to unity. Between Rey ≌ 50 and Rey ≌ 1000, Cp is slightly less than unity, but has a minimum value of 0.99. For values of Rey less than 50, Cp is always greater than unity. When the Reynolds number is below unity, the pressure rise is independent of the fluid density, and the data may be represented approximately by the formula Cp ≌ 5.6/Rey. The results are compared with the experimental investigations of Barker and of Homann, and with the theoretical studies of Stokes and of Homann.

An experimental investigation of turbulent spots and breakdown to turbulence

1960 ◽  
Vol 9 (2) ◽  
pp. 235-246 ◽  
Author(s):  
J. W. Elder
Keyword(s):  
Boundary Layer ◽  
Experimental Investigation ◽  
Reynolds Number ◽  
Hydrodynamic Stability ◽  
Free Stream ◽  
Stream Velocity ◽  
Turbulent Spot ◽  
Turbulent Spots ◽  
The Impact ◽  
Critical Intensity

The theory of hydrodynamic stability and the impact on it of recent work with turbulent spots is discussed. Emmons's (1951) assumptions about the growth and interaction of turbulent spots are found experimentally to be substantially correct. In particular it is shown that the region of turbulent flow on a flat plate is simply the sum of the areas that would be obtained if all spots grew independently.An investigation of the conditions required for breakdown to turbulence near a wall, that is, to initiate a turbulent spot, suggests that regardless of how disturbances are generated in a laminar boundary layer and independent of both the Reynolds number and the spatial extent of the disturbances, breakdown to turbulence occurs by the initiation of a turbulent spot at all points at which the velocity fluctuation exceeds a critical intensity. Over most of the layer this intensity is about 0·2 times the free-stream velocity. The Reynolds number is important merely in respect of the growth of disturbances prior to breakdown.

Experimental Study on the Effect of Reynolds Number Variation on Drag Force for Various Cut Angle on D-Type Cylinders

2014 ◽  
Vol 493 ◽  
pp. 140-144
Author(s):  
Astu Pudjanarsa ◽  
Ardian Ardawalika
Keyword(s):  
Experimental Study ◽  
Reynolds Number ◽  
Drag Force ◽  
Free Stream ◽  
Force Balance ◽  
Stream Velocity ◽  
Turning Angle ◽  
Subsonic Wind Tunnel ◽  
Minimum Drag ◽  
Number Variation

Experimental study on the effect of Reynolds number variation on drag force for various cut angles on D-type cylinders was performed. Five different cut angles on different cylinders were applied including: 35o, 45o, 53o, 60o, and 65o. The free stream velocity was varied so the Reynolds number also varied.The experiment was carried out at a subsonic wind tunnel. Drag force for a cut D-type cylinder (for example 35o) was measured using a force balance and wind speed was varied so that corresponding Reynolds number of 2.4×104÷5.3×104 were achieved. Wind turning angle was kept at 0o (without turning angle). This experiment repeated for other D-type cylinders.Experiment results show that, for all D-type cylinders, drag force decreased as the Reynolds number increased, then it was increased after attain minimum drag force. For all D-type cylinders and all variations of Reynolds number the drag minimum is attained at cut angle of 53o. This value is appropriate with previous experiment results.

CFD Study of the Pressure Distribution on the Back of a 3-D Bluff Body (CUBE) of Air Flow in Different Reynolds Numbers

2009 ◽  
Author(s):  
Mohammad Javad Izadi ◽  
Pegah Asghari ◽  
Malihe Kamkar Delakeh
Keyword(s):  
Reynolds Number ◽  
Bluff Body ◽  
Free Stream ◽  
Fluid Flows ◽  
Reynolds Numbers ◽  
The Body ◽  
Stream Velocity ◽  
Bluff Bodies ◽  
Unsteady Forces ◽  
Flow Round

The study of flow around bluff bodies is important, and has many applications in industry. Up to now, a few numerical studies have been done in this field. In this research a turbulent unsteady flow round a cube is simulated numerically. The LES method is used to simulate the turbulent flow around the cube since this method is more accurate to model time-depended flows than other numerical methods. When the air as an ideal fluid flows over the cube, flow separate from the back of the body and unsteady vortices appears, causing a large wake behind the cube. The Near-Wake (wake close to the body) plays an important role in determining the steady and unsteady forces on the body. In this study, to see the effect of the free stream velocity on the surface pressure behind the body, the Reynolds number is varied from one to four million and the pressure on the back of the cube is calculated numerically. From the results of this study, it can be seen that as the velocity or the Reynolds number increased, the pressure on the surface behind the cube decreased, but the rate of this decrease, increased as the free stream flow velocity increased. For high free stream velocities the base pressure did not change as much and therefore the base drag coefficient stayed constant (around 1.0).

Viscous Scaling Phenomena in Miniature Centrifugal Flow Cooling Fans: Theory, Experiments and Correlation

2010 ◽  
Vol 132 (2) ◽  
Author(s):  
Patrick A. Walsh ◽  
Edmond J. Walsh ◽  
Ronan Grimes
Keyword(s):  
Reynolds Number ◽  
Scale Effects ◽  
Pressure Coefficient ◽  
Pressure Rise ◽  
Chord Length ◽  
Flow Coefficient ◽  
Power Coefficient ◽  
Small Scale ◽  
Cooling Fans ◽  
Novel Theory

This paper analyzes the scale effects that occur in miniature centrifugal flow fans and investigates the possibility of optimizing blade geometry so that performance can be enhanced. Such fans are typically employed in small scale heat sinks such as those used for processor cooling applications or in portable electronics. The specific design parameter varied is the blade chord length, and the resulting fan performance is gauged by examining the flow rate, pressure rise, and power consumption characteristics. The former two are measured using a BS 848 fan characterization rig and the latter, by directly measuring the power consumed. These characteristics are studied for three sets of scaled fans with diameters of 15 mm, 24 mm, and 30 mm, and each set considers six individual blade chord lengths. A novel theory is put forward to explain the anticipated effect of changing this parameter, and the results are analyzed in terms of the relevant dimensionless parameters: Reynolds number, chord length to diameter of fan ratio, flow coefficient, pressure coefficient, and power coefficient. When these characteristic parameters are plotted against the Reynolds number, similar trends are observed as the chord length is varied in all sets of scaled fans. The results show that the flow coefficient for all the miniature fans degrade at low Re values, but the onset of this degradation was observed at higher Re values for longer blade chord designs. Conversely, it was found that the pressure coefficient is elevated at low Re, and the onset Re for this phenomenon correlates well with the drop off in flow coefficient. Finally, the trend in power coefficient data is similar to that for the flow coefficient. The derived theory is used to correlate this data for which all data points fall within 6% of the correlation. Overall, the findings reported herein provide a good understanding of how changing the blade chord length affects the performance of miniature centrifugal fans; hence, providing fan designers with guidelines to aid in developing optimum blade designs, which minimize adverse scaling phenomena.

Experimental Investigation of Free Stream Turbulence Effects on Flow Separation

2003 ◽  
Author(s):  
Mahmoud Ardebili ◽  
Yiannis Andreopoulos
Keyword(s):  
Boundary Layer ◽  
Experimental Investigation ◽  
Reynolds Number ◽  
Pressure Gradient ◽  
Flow Separation ◽  
Large Scale ◽  
Free Stream ◽  
Pressure Coefficient ◽  
Free Stream Turbulence ◽  
Vorticity Flux

An experimental investigation of a separated boundary layer flow has been attempted which has been created by perturbing a flat plate flow with a favorable pressure gradient immediately followed by an adverse pressure gradient. The aim of the research program is possible control of flow separation by means of free stream turbulence. The flow is configured in a large-scale low speed wind tunnel where measurements of turbulence can be obtained with high spatial and temporal resolution. A model has been designed by using CFD analysis. Mean wall pressure and vorticity flux measurements are reported in this paper. Twelve experiments with three different mesh size grids at three different Reynolds numbers have been carried out. Three bulk flow parameters seem to characterize the flow: The Reynolds number of the boundary layer, Re+, the Reynolds number of the flow through the grid, ReM, and the solidity of the grid. It was found that the pressure coefficient depends weakly on the solidity of the grids. Vorticity flux also depends on the grid used to generate free stream turbulence. The location of maximum or minimum vorticity flux moves upstream at higher ReM.

Reynolds Number Dependence of the Flow Structure Behind Two Side-by-Side Cylinders

2002 ◽  
Author(s):  
S. J. Xu ◽  
Y. Zhou ◽  
R. M. C. So
Keyword(s):  
Reynolds Number ◽  
Flow Structure ◽  
Free Stream ◽  
Present Finding ◽  
Dominant Frequency ◽  
Wind Tunnels ◽  
Stream Velocity ◽  
Single Vortex ◽  
The One ◽  
Reynolds Number Dependence

The wake structure of two side-by-side cylinders was experimentally investigated using flow visualization and hotwire techniques. The investigation was focused on the asymmetrical flow regime, i.e., T/d = 1.2 – 1.6, where T is the center-to-center cylinder spacing and d is the cylinder diameter. Experiments were conducted in both water and wind tunnels at a Reynolds number (Re) range of 150 – 14300. It has been found that, as Re increases, the flow structure behind the cylinders would change from one single vortex street to two streets with one narrow and one wide, for the same T/d. The one-street flow structure is dominated by one frequency ƒ0* = ƒ0d/U∞ ≈ 0.09, where ƒ0 is the dominant frequency and U∞ is the free-stream velocity. On the other hand, two frequencies, ƒ0* ≈ 0.3 and 0.09, characterized the two-street flow structure. These are associated with the narrow and wide street frequency, respectively. It is further observed that the critical Re, at which transition from single to two streets occurs, increases as T/d decreases. The present finding help clarify previous scattered reports for 1.2 < T/d < 1.5: detection of one dominant frequency by some but two by others.

A Closed Formula for the Drag of a Flat Plate with Transition in the Absence of a Pressure Gradient

1960 ◽  
Vol 64 (589) ◽  
pp. 38-39 ◽  
Author(s):  
A. R. Collar
Keyword(s):  
Reynolds Number ◽  
Pressure Gradient ◽  
Flat Plate ◽  
Transition Point ◽  
Free Stream ◽  
Analytical Form ◽  
Stream Velocity ◽  
Closed Formula ◽  
Friction Drag ◽  
Do So

Most University students of fluid mechanics are familiar with the problem of the evaluation of the skin friction drag of a flat plate in the absence of a pressure gradient: transition is specified, usually by the free stream velocity and the transition point, or directly by the transition Reynolds number. The solution is normally obtained numerically: so far as the writer is aware, the processes used have not been written down in closed analytical form, though to do so presents no difficulty.

Unsteady drag on a sphere at finite Reynolds number with small fluctuations in the free-stream velocity

1991 ◽  
Vol 233 ◽  
pp. 613-631 ◽  
Author(s):  
Renwei Mei ◽  
Christopher J. Lawrence ◽  
Ronald J. Adrian
Keyword(s):  
Reynolds Number ◽  
Free Stream ◽  
Low Frequency ◽  
Reynolds Numbers ◽  
Free Stream Velocity ◽  
Creeping Flow ◽  
Stream Velocity ◽  
Velocity Difference ◽  
Long Time ◽  
The Time Domain

Unsteady flow over a stationary sphere with small fluctuations in the free-stream velocity is considered at finite Reynolds number using a finite-difference method. The dependence of the unsteady drag on the frequency of the fluctuations is examined at various Reynolds numbers. It is found that the classical Stokes solution of the unsteady Stokes equation does not correctly describe the behaviour of the unsteady drag at low frequency. Numerical results indicate that the force increases linearly with frequency when the frequency is very small instead of increasing linearly with the square root of the frequency as the classical Stokes solution predicts. This implies that the force has a much shorter memory in the time domain. The incorrect behaviour of the Basset force at large times may explain the unphysical results found by Reeks & Mckee (1984) wherein for a particle introduced to a turbulent flow the initial velocity difference between the particle and fluid has a finite contribution to the long-time particle diffusivity. The added mass component of the force at finite Reynolds number is found to be the same as predicted by creeping flow and potential theories. Effects of Reynolds number on the unsteady drag due to the fluctuating free-stream velocity are presented. The implications for particle motion in turbulence are discussed.

The Magnus Effect: A Summary of Investigations to Date

1961 ◽  
Vol 83 (3) ◽  
pp. 461-470 ◽  
Author(s):  
W. M. Swanson
Keyword(s):  
Reynolds Number ◽  
Free Stream ◽  
Mathematical Problem ◽  
Experimental Results ◽  
Stream Velocity ◽  
Magnus Force ◽  
Drag Forces ◽  
Number Ratio ◽  
Magnus Effect ◽  
Lift And Drag

The Magnus force on a rotating body traveling through a fluid is partly responsible for ballistic missile and rifle shell inaccuracies and dispersion and for the strange deviational behavior of such spherical missiles as golfballs and baseballs. A great deal of effort has been expended in attempts to predict the lift and drag forces as functions of the primary parameters, Reynolds number, ratio of peripheral to free-stream velocity, and geometry. The formulation and solution of the mathematical problem is of sufficient difficulty that experimental results give the only reliable information on the phenomenon. This paper summarizes some of the experimental results to date and the mathematical attacks that have been made on the problem.

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