Kazuhiko Yokota

Flutter Limits and Behavior of a Flexible Thin Sheet in High-Speed Flow— II: Experimental Results and Predicted Behaviors for Low Mass Ratios

Flutter phenomena of flexible thin sheets such as paper swept by wind are studied for a wide range of mass ratios by an analytical method developed by the authors. The analytical results explain well the tendencies found in the experimental data. Furthermore, the flutter behaviors for very low mass ratios are predicted to tend to deviate far from those for ordinarily stiff materials, which are attributed to the effects of both the fluid friction and the added mass effect by surrounding fluid in addition to the ordinary governing effects for higher mass ratios, i.e., elasticity, inertia force, and fluid pressure.

Flutter Limits and Behaviors of a Flexible Thin Sheet in High-Speed Flow— I: Analytical Method for Prediction of the Sheet Behavior

An analytical method is proposed for the prediction of fluttering of a flexible thin sheet or web swept by fluid flow. It assumes self-excited aeroelastic oscillation of the sheet with infinitesimally small amplitude. The flow and the sheet motion are expressed by distributed vortices over the sheet and the wake, and by bending motions of a number of short segments constituting the sheet. The obtained system of equations determines the flutter limits, the oscillation modes and frequencies. The method treats particularly well the situation of very low mass ratios where the modes are far from those in vacuum and progressive waves are predominant.

Drag reduction in a turbulent boundary layer on a flexible sheet undergoing a spanwise traveling wave motion

The effect of a spanwise traveling-wave motion on a zero-pressure-gradient turbulent boundary layer over a flexible sheet was investigated at low Reynolds numbers using a single hot-wire anemometer for turbulence statistics and two laser displacement sensors for displacements of the flexible sheet. It was found that the log-law region of the mean velocity on the flexible sheet was slightly narrower compared with a rigid wall. The energy spectra of streamwise velocity fluctuations on the flexible sheet undergoing the spanwise traveling-wave motion were smaller in a region of frequency which corresponded to the bursting frequency in the canonical wall turbulence. This indicates that the bursting event near the flexible sheet was directly affected by the surface wave motion. It was revealed that a drag reduction of up to 7.5% could be obtained by the spanwise traveling-wave motion, estimating the friction coefficients through the growth rate of the momentum thickness.

Turbulent drag reduction by the seal fur surface

The drag-reducing ability of the seal fur surface was tested in a rectangular channel flow using water and a glycerol-water mixture to measure the pressure drop along the channel in order to evaluate friction factors in a wide range of Reynolds number conditions, and the drag reduction effect was confirmed quantitatively. The maximum reduction ratio was evaluated to be 12% for the glycerol-water mixture. The effective range of the Reynolds number, where the drag reduction was remarkable, was wider for the seal fur surface compared to that of a riblet surface measured in this channel and in previous studies. It was also found that for the seal fur surface, unlike riblets, any drag increase due to the effect of surface roughness was not found up to the highest Reynolds number tested. Measurements of the seal fur surface using a 3D laser microscope revealed that there were riblet-like grooves, composed of arranged fibers, of which spacings were comparable to that of effective riblets and were distributed in ...

Velocity measurement in turbulent boundary layer of drag-reducing surfactant solution

The influence of a drag-reducing surfactant on a zero-pressure gradient turbulent boundary layer was investigated using a two-component laser-Doppler velocimetry system. It was discovered that the streamwise turbulence intensity has a maximum near the center of the boundary layer in addition to the near-wall maximum which appears in canonical wall-bounded turbulent flow. At the location of the additional maximum, the Reynolds shear stress has a slight maximum, the skewness factor of streamwise turbulent fluctuation is zero, and the flatness factor has a minimum.

Turbulent drag reduction in nonionic surfactant solutions

There are only a few studies on the drag-reducing effect of nonionic surfactant solutions which are nontoxic and biodegradable, while many investigations of cationic surfactant solutions have been performed so far. First, the drag-reducing effects of a nonionic surfactant (AROMOX), which mainly consisted of oleyldimethylamineoxide, was investigated by measuring the pressure drop in the pipe flow at solvent Reynolds numbers Re between 1000 and 60 000. Second, we investigated the drag-reducing effect of a nonionic surfactant on the turbulent boundary layer at momentum-thickness Reynolds numbers Reθ from 443 to 814 using two-component laser-Doppler velocimetry and particle image velocimetry systems. At the temperature of nonionic surfactant solutions, T=25 °C, the maximum drag reduction ratio for AROMOX 500 ppm was about 50%, in the boundary layer flow, although the drag reduction ratio was larger than 60% in pipe flow. Turbulence statistics and structures for AROMOX 500 ppm showed the behavior of typical dr...

Direct numerical simulation of the drag-reducing turbulent boundary layer of viscoelastic fluid

Direct numerical simulation of a zero-pressure gradient drag-reducing turbulent boundary layer of homogeneous viscoelastic fluids was performed using constitutive equation models such as the Oldroyd-B and Giesekus models. Mean velocity profiles and turbulence statistics at the different streamwise locations were discussed using both inner and outer scaling. The maximum drag reduction ratio for the Oldroyd-B model, which has the higher elongational viscosity, is larger than for the Giesekus model. The distinct difference in turbulence statistics near the wall between the Oldroyd-B model and Newtonian fluid is observed, as reported in the drag-reducing turbulent channel flow, while in the outer region, distributions of turbulence statistics for the Oldroyd-B model with a drag reduction ratio of about 40% are similar to those for Newtonian fluid. The production term for the turbulent boundary layer does not correspond to the amount of drag reduction, which is consistent with the fact that the streamwise turb...

Turbulence statistics and structures of drag-reducing turbulent boundary layer in homogeneous aqueous surfactant solutions

In our earlier work [Itoh et al., Phys. Fluids 17, 075107 (2005)], the additional maximum of the streamwise turbulence intensity near the center of the drag-reducing turbulent boundary layer was found in the homogeneous dilute aqueous surfactant solution which was a mixture of cetyltrimethyl ammonium chloride with sodium salicylate as counterion. In this work, we systematically investigated the influence of the drag-reducing surfactant on the velocity fields of the turbulent boundary layer at various Reynolds numbers Reθ from 301 to 1437 and the drag reduction ratio DR from 8% to 74% under different streamwise locations and concentration and temperature of solutions using a two-component laser-Doppler velocimetry (LDV) system. It was revealed that all data on DR versus the wall-shear rate obtained here were collapsed on a single curve. We verified the existence of the additional maximum of the streamwise turbulence intensity near the center of the boundary layer which appeared at relatively large drag red...

Three Dimensional Unsteady Flow Simulation of Compressed Truncated Perfect Nozzles

Recently, a large side load was observed during the startup and shutdown transients of the Japanese LE-7A rocket engine nozzle. The objective of the present paper is to clarify the mechanism producing the large side loads during the startup transient. The unsteady three-dimensional Navier-Stokes equations are solved for the startup transients of the three types of nozzle contours – the Truncated Perfect (TP) nozzle, the 86% Compressed Truncated Perfect (CTP) nozzle and the CTP50-R5-L nozzle. The contour of the TP nozzle is the LE-7 nozzle and the contour of the 86% CTP nozzle is the LE-7A nozzle. The CTP50-R5-L nozzle was tested by Tomita et al and Takahashi et al . The asymmetric flow patterns and the large side loads are observed in LE-7A and CTP50-R5-L nozzle. There are two types of flow patterns that cause the large side loads. The first is the simultaneous occurrence of the Free Shock Separation (FSS) and the Restricted Shock Separation (RSS). The second is the oscillation of shock wave under RSS condition . The interaction between the internal shock and the Mach disk causes the cap shock and the trapped vortex. For both flow patterns, the trapped vortex oscillates with the cap shock. The trapped vortex is developed with oscillation as the Nozzle Pressure Ratio (NPR) increases. At a certain range of the NPR, oscillations of the cap shock and the trapped vortex become larger, and then the supersonic jet on the both sides of the cap shock reattaches to the nozzle wall partially –the FSS and the RSS occur simultaneously. The second flow pattern is observed in CTP50-R5-L nozzle. With the RSS, the cap shock oscillates with the trapped vortex. These flow patterns produce the asymmetric wall pressure distribution and the large side load. Nomenclature p : Pressure R : Gas constant γ : Specific heat ratio M : Mach number NPR : Nozzle Pressure Ratio, a c p p / Subscript c : combustion chamber a : ambient Introduction Several new rocket engines in the world have the new-shaped nozzle instead of the conventional Truncated Perfect (TP) nozzle. For example, the thrust-optimized nozzles are applied for the American Space Shuttle Main Engine (SSME) and the European Vulcain. On the other hand, the Japanese LE-7A nozzle is designed as the Compressed Truncated Perfect (CTP) nozzle. In these nozzles, the large side loads are observed during the startup and shutdown transients at the sea level firing test. The side loads are undesirable since it may damage the engine support system. The side loads in rocket nozzles have been the subjects of various studies, but the detailed flow structures and the mechanisms of the side loads have not been understood completely. The flow separates from the nozzle wall under the low chamber pressure condition during the startup and shutdown transients. It is considered that the behavior of the separated flow influences on the side load. *Graduate Student, Graduate School of Engineering Science, Osaka University †Associate Professor, Department of Mechanical Engineering, Nagoya Institute of Technology, Member AIAA ‡Professor, Graduate School of Engineering Science, Osaka University, Member AIAA §Senior Engineer, NASDA, Member, AIAA ¶Associate Senior Engineer, NASDA, Member AIAA 38th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit 7-10 July 2002, Indianapolis, Indiana AIAA 2002-3991 Copyright

Modeling of 2-D leakage jet cavitation as a basic study of tip leakage vortex cavitation

As the first step of the study of cavitation in the tip leakage flow in turbomachinery, experimental and numerical investigations on characteristics of 2-D leakage jet cavitation are conducted. The experiments using 2-D model apparatus are carried out for three values of initial pressure and for three values of tip clearance. The numerical methods are constructed by combining the vortex method with three types of cavity models