Yasuaki Kohama

Spiral vortices in boundary layer transition regime on a rotating disk

SummaryBehavior of spiral vortices on a disk rotating in still fluid is studied theoretically and experimentally in detail. A linear stability analysis, in which effects of streamline curvature and Coriolis force are considered, gives a critical Reynolds number at the onset of instability close to the one measured here by using a hot wire probe. Gradient of the vortex axis is determined under a condition of the maximum amplification. Flow patterns in the transition regime are experimentally visualized. The results show that the number of the spiral vortices is 31 or 32 as mean value and the gradient of the vortex axis decreases from 14° to 7° as the local Reynolds number is increased.ZusammenfassungDas Verhalten der spiralen Wirbel an einer in ruhender Flüssigkeit rotierenden Scheibe wird theoretisch und experimentell eingehend untersucht. Eine lineare Stabilitätstheorie, in welcher der Einfluß von Krümmung der Stromlinien und Coriolis-Kraft in Betracht gezogen wird, ergibt eine kritische Reynolds-Zahl des Indifferenzpunktes, die gut mit dem mit einem Hitzdrahtgerät gemessenen Wert übereinstimmt. Der Gradient der Wirbelachse wird unter der Bedingung der maximalen Anfachung bestimmt. Der Strömungsvorgang im Umschlagbereich wird experimentell sichtbar gemacht. Dadurch ergibt sich, daß die Anzahl der an der Scheibe auftretenden Wirbel 31 oder 32 im Mittel beträgt, und der Gradient der Wirbelachse mit zunehmender lokaler Reynolds-Zahl von 14° bis zu 7° abnimmt.

Study on boundary layer transition of a rotating disk

SummaryBehaviour of spiral vortices being generated in transition regime of a disk rotating in otherwise undisturbed fluid is experimentally studied in detail. Through visualizations of the transition regime by using close-up camera, new striped flow patterns originating along the axis of spital vortices are found to be ring-like vortices which occur on the surfaces of each spiral vortices. Mechanism of the spiral vortex is clarified by cutting the vortices by strobo slit light. It is also found out experimentally that the phase velocity of the vortices is zero.

Aerodynamics of a NACA4412 airfoil in ground effect

The flow characteristics over a NACA4412 airfoil are studied in a low turbulence wind tunnel with moving ground simulation at a Reynolds number of 3.0 x 105 by varying the angle of attack from 0 to 10 deg and ground clearance of the trailing edge from 5% of chord to 100%. The pressure distribution on the airfoil surface was obtained, velocity survey over the surface was performed, wake region was explored, and lift and drag forces were measured. To ensure that the flow is 2-D, particle image velocimetry measurements were performed. A strong suction effect on the lower surface at an angle of attack of 0 deg at the smallest ground clearance caused laminar separation well ahead of the trailing edge. Interestingly, for this airfoil, a loss of upper surface suction was recorded as the airfoil approached the ground for all angles of attack. For angles up to 4 deg, the lift decreased with reducing ground clearance, whereas for higher angles, it increased due to a higher pressure on the lower surface. The drag was higher close to the ground for all angles investigated mainly due to the modification of the lower surface pressure distribution.

Boundary-layer transition on a rotating cone in axial flow

The purpose of the present paper is to investigate the structure of the laminar–turbulent transition region for the three-dimensional boundary layer along a 30° cone rotating in external axial flow. Spiral vortices, which were assumed as small disturbances in the present stability analysis, are observed experimentally in the transition region. The process of transition to a turbulent boundary layer is visualized in detail. When the ratio of rotational speed to external axial flow is increased, the critical and transition Reynolds numbers decrease remarkably. The spiral angle and the number of vortices appearing on the cone decrease as the rotational speed ratio is increased.

Boundary-layer transition and the behaviour of spiral vortices on rotating spheres

Etude experimentale du mecanisme de transition de la couche limite et du comportement de tourbillons en spirale sur des spheres tournantes dans un fluide par ailleurs non perturbe. Mesure des nombres de Reynolds critique et de transition determinant le regime de transition laminaire turbulente a la surface de la sphere

Some expectation on the mechanism of cross-flow instability in a swept wing flow

SummaryThree-dimensional boundary layer transition on axisymmetric rotating bodies is the subject of a comprehensive experimental study. Based on this study, hypotheses are made on the mechanism of cross-flow instability for swept wing flow. These new results are combined with past explanations to provide a rough sketch for the entire flow field over the swept wing. From this new viewpoint there appears the mechanism of traveling waves, being induced by a stationary disturbance. Some uncertainties appearing in recent papers concerning this flow field are discussed. Among these uncertainties for which an explanation is provided, is the discrepancy of frequencies between the hot wire signal and the visualized flow pattern.

Design and Control of Crossflow Instability Field

In order to find out the transition mechanism of crossflow dominant boundary layers in detail, experimental model which is composed of yawed flat plate with displacement upper body, is designed and full transition process from onset of transition to fully turbulent state is generated in the flat plate boundary layer Systematic measurement is conducted on this crossflow instability field using hot wire velocimetry with accurate traversing mechanism and effective flow visualizations. Results show that such a complicated flow condition, where several different disturbances are appearing and interacting, is occurring in the transition region. Such flow condition is quite similar to that of swept wing boundary layer flow. In the transition region, stationary crossflow vortices, crossflow instability unsteady mode, high frequency secondary instability mode are also measured. Schematic sketch of the obtained flow field where these instabilities are most amplified is drown. Directions and phase velocities of obtained disturbances are also measured, and indicated in one figure together with twisted boundary profiles. Secondary instability is successfully visualized by smoke visualization technique, and physical structure of the secondary instability is compared with the results of hot wire measurement. Good agreement is obtained especially for the travel direction of the secondary instability, discussions are also made concerning obtained interesting results.

Behaviour of spiral vortices on a rotating cone in axial flow

SummaryThe purpose of the present paper is to investigate experimentally in detail the boundary layer transition process and the behaviour of spiral vortices appearing in the transition range of the boundary layer on a 30°-cone, rotating in axial flow. Counterrotating spiral vortices in the transition range are visualized with a white smoke method, and observed the time dependent behaviour of them using a drum camera and a light sheet illumination method with a stroboscope flash light. The light passes a slit in order to illuminate only a thin sheet in the flow. With this method, the time dependent growing up and breaking down process of these spiral vortices is greatly clarified. A hot wire anemometer is also used for measuring in the flow field quantitatively. The results show that the spiral vortices are generated in the thin region of the steep shear velocity gradients near the wall. As the vortices grow up in z-direction, they are strongly distorted by the mean velocity field there, and finally they are teared off.

Crossflow instability in a spinning disk boundary layer

Nomenclature D = diameter of a disk k = constant TV = spinning speed of a disk, rpm n = number of crossflow vortices Re = Reynolds number based on spinning speed r — distance along radius Tu = turbulence intensity U = velocity u = circumferential velocity in the boundary layer v = radial velocity in the boundary layer x = coordinate axis in freestream direction y = coordinate axis in azimuthal direction z = coordinate axis normal to the wall X = wavelength of disturbances X = crossflow parameter

Three-dimensional Boundary Layer Transition on a Concave-Convex Curved Wall

The flow structure of the pressure side of a super critical LFC wing (NASA#998A), which has a concave-convex curvature at the leading edge region, is studied experimentally in detail. The flow field is very complicated being suffered from centrifugal force with streamline curvatures both in parallel and perpendicular to the wall surface. As a result, complicated 3-D boundary-layer flow with several kinds of instabilities like Taylor-Gortler instability, crossflow instability and Tollmien-Schlichting instability are expected.