Cross-stream Pressure Gradient Research Articles

Investigation of turbulent separation-reattachement flow in a curved-wall diffuser

A two-dimensional (2-D) laser velocimeter was employed to study the turbulent separation-reattachment flow field in a 2-D diffuser, which is a lower curved wall and upper flat plate. There is a parallel channel connected with the exit of the diffuser. In the inlet of the diffuser, the Reynolds number is 5,000 based on the momentum thickness, and the inlet velocity is 25.2 m/s. Mean velocity and Reynolds stresses were measured from upstream of the separation to downstream of the reattachment. The minimum distance from the surface is 0.3 mm. The significant features were that after transitory detachment, within the reversing flow, there exist the second extreme of u 2 and minus of — uv . Normal stresses and the cross-stream pressure gradient are important immediately in the separating flow and are associated with strong streamline curvature. The maximum of the displacement thickness curvature K max ∗ corresponds to the intermittency transitory detachment. Several velocity profiles and Cebeci and Smith algebraic eddy-viscosity are compared with the experiment. A new approximate correction of the effect of normal stress is proposed and yields results in agreement with the experiment before the transitory detachment.

The structure of a three-dimensional turbulent boundary layer

The three-dimensional turbulent boundary layer is shown to have a self-consistent two-layer asymptotic structure in the limit of large Reynolds number. In a streamline coordinate system, the streamwise velocity distribution is similar to that in two-dimensional flows, having a defect-function form in the outer layer which is adjusted to zero at the wall through an inner wall layer. An asymptotic expansion accurate to two orders is required for the cross-stream velocity which is shown to exhibit a logarithmic form in the overlap region. The inner wall-layer flow is collateral to leading order but the influence of the pressure gradient, at large but finite Reynolds numbers, is not negligible and can cause substantial skewing of the velocity profile near the wall. Conditions under which the boundary layer achieves self-similarity and the governing set of ordinary differential equations for the outer layer are derived. The calculated solution of these equations is matched asymptotically to an inner wall-layer solution and the composite profiles so formed describe the flow throughout the entire boundary layer. The effects of Reynolds number and cross-stream pressure gradient on the cross-stream velocity profile are discussed and it is shown that the location of the maximum cross-stream velocity is within the overlap region.

An experimental investigation into the effects of rotation on the isothermal flow resistance in circular tubes rotating about a parallel axis

Results are presented of an experimental investigation into the influence of rotation on the flow resistance of a circular sectioned tube rotating about and axis parallel to its central axis of symmetry. This investiation is part of a long-term study intop the effect of rotation on pressure loss and heat transfer characteristics in rotating coolant channels. Analytical examination of the governing conservation equations indicate that for isothermal flow rotation will have no effect on the fully developed flow apart from a cross-stream hydrostatic pressure gradient due to additional centripetal acceleration terms in the equations; this has been verified experimentally. In the entrance region the effects of the additional centripetal acceleration terms would be manifest solely as a hydrostatic pressure field; this was also verified experimentally. In the entrance region analytical examination of the governing conservation equations predict Coriolis-induced secondary flows. The existence of Coriolis-induced effects in the entrance region have been experimentally verified, increases in the friction factor being noted that were correlated using the rotational Reynolds number, which is a measure of the relative strength of Coriolis to viscous forces.

The Influence of Curvature on Film Cooling Performance

The effects of injection rate and strength of curvature on film cooling performance of gas injected through a row of holes on a convex surface are studied. Comparisons are made to film cooling of concave and flat surfaces. Three different relative strengths of curvature (ratio of radius of curvature to radius of injection hole), two density ratios (0.95 and 2.0), and a wide range of blowing rates (0.3 to 2.7) are considered. A foreign gas injection technique (mass transfer analogy) is used. The strength of curvature was controlled by varying the injection hole diameter. At low blowing rates, film cooling is more effective on the convex surface than on a flat or a concave surface. The cross-stream pressure gradient present in curved flows tends to push the jet into the convex wall. As the injection rate is increased, normal and tangential jet momentum promote liftoff from the convex surface, thereby lowering performance. In contrast, previous studies show that on a concave surface, tangential jet momentum, flow instabilities, and blockage improve performance on a concave surface as blowing rate is increased.

Why a corrugated coast only influences a longshore current within a wave length of the coast

An inviscid zone of influence is postulated from a physical model to occur when a steady homogeneous fluid flows by a two-dimensional corrugated wall with small slopes. The thickness of the influence zone is calculated to be a constant times the wave length of the wall. The method used involves balancing pressure differences between extreme points along the wall and between the wall and the outer edge of the influence zone but does not involve the irrotational assumption. An influence zone forms because the curvature of the boundary induces cross-stream accelerations and pressure gradients in the fluid. The result for the thickness of the influence zone agrees qualitatively with the exponential decay scale of an available potential flow solution to a similar problem, which suggests that the influence zone may occur in some rotational (as well as irrotational) flows.

Characteristics of a trailing-edge flow with turbulent boundary-layer separation

Experimental techniques, including flying-hot-wire anemometry, have been used to determine the pressure and velocity characteristics of a flow designed to simulate the trailing-edge region of an airfoil at high angle of attack. Emphasis is placed on the region of recirculating flow and on the downstream wake. It is shown that the effect of this recirculation is large even though the details of the flow within it may be unimportant. Normal stresses and cross-stream pressure gradients are important immediately upstream and downstream of the recirculating flow and are associated with strong streamline curvature. The relative importance of the terms in the transport equations for mean momentum and turbulence energy are quantified and the implications for procedures which solve potential-flow and boundary-layer equations and for alternative calculation methods are discussed.

On the mechanics of the deep-water flow in the Bornholm Channel

A model for stationary and longitudinally homogeneous 2-layer rotating canal flow subject to friction is proposed for the deep-water flow in the Bornholm Channel. Using parameter values estimated from the results of a field survey carried out in conjunction with a long term program for monitoring the inflow of high-saline water to the Baltic, diagnostic calculations for the position of the pycnocline have been performed. The results correspond to an encouraging degree to those observed in nature and indicate the existence ofa frictional coupling between the down-and cross-stream pressure gradients in deep-water. DOI: 10.1111/j.1600-0870.1983.tb00192.x

THREE-DIMENSIONAL LAMINAR FLOW IN DUCTS

A finite-element marching procedure is presented for the calculation of transport processes in three-dimensional parabolic flows As in corresponding finite-difference procedures, equations are solved one after the other. Similarly, longitudinal and cross-stream pressure gradients are uncoupled. Velocity fields are determined by first calculating intermediate velocity values based on estimated pressure gradient distributions and then obtaining appropriate corrections so as to satisfy the continuity equation Illustrative examples concerning flow in the entrance region of straight ducts demonstrate the accuracy and reliability of the proposed technique.

Some Aspects of the Quasi-Geostrophic Flow over Mountains

Abstract Previous models of quasi-geostrophic airflow over mountains have used one or more of the following assumptions: 1) the Boussinesq approximation, 2) a rigid lid upper boundary condition, and 3) the idealization of an infinitely long ridge and two-dimensional flow. To investigate the physical nature of these assumptions, the flow of an unbounded, slightly compressible fluid over an isolated mountain is considered. In this case, the mountain feels a strong “lift” force acting to the left of the stream (in the Northern Hemisphere) due to the cross-stream pressure gradient, while the airstream receives an equally strong impulse to the right. Using the Kutta-Joukowski formula, the rightward impulse given to the airstream can be represented in terms of a far-field barotropic circulation which in turn is associated with the production of vorticity by volume expansion as the air parcels rise over the mountain.

Supersonic interference flow along the corner of intersecting wedges.

Abstract : An experimental investigation of the inviscid supersonic flow field in a corner formed by two intersecting wedges is reported. The study includes measurements of the distribution of the surface pressure and the wave structure of the flow field in the corner region both for a symmetrical corner configuration (e. e., equal wedge angles) in the Mach-number range between 2.5 and 4 and for an asymmetrical model at one Mach number as a function of the incidence angle of one wedge. A clearly identifiable shock structure is discovered that divides the interference field into four zones. The total extent of the disturbance near the surfaces of the model is approximately twice the extent indicated by available small-perturbation analyses. The highest pressure rise occurs in the inner zone near the corner; the magnitude of this rise is related successfully to the second-order linearized theory. The inviscid flow field is shown to involve strong cross-stream pressure and entropy gradients both at the corner proper and away from it, which cannot be disregarded in analyses of the viscous boundary layer. (Author)