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Home Aerospace & Aeronautics Shapes without separation and cavitation

Shapes without separation and cavitation

Description

The pressure distribution over the surface is the main reason of separation. In turn, unseparated flow patterns can prevent the cavitation inception. Therefore, the shapes with a special pressure distribution can prevent both separation and cavitation. Theoretical results and wind tunnel tests showed that axisymmetric shapes with negative pressure gradients over the surface can be calculated and used to prevent separation.

An example of such shape (the body U-1) is presented in Pic. 1. The body U-1 revealed no separation at higher performance velocities (Reynolds numbers grater than 220000). The shape UA-2 en-sured an unseparated flow pattern at all Reynolds numbers available in the wind tunnel of Kyiv Institute of Hydromechanics. Progress in design of the axisymmetric unseparated shapes aroused interest in appropriate 2D forms. Symmetric profiles similar to the axisymmetric bodies with the negative pressure gradients over the surface were also desighed. The calculations show that the region with a positive pressure gradient can be very short both for thin and thick profiles.

Such shapes are of both theoretical and practical interest. In particular, according to the standard opinion, there must be a separation at or downstream of the minimum pressure point. Nevertheless, the absence of separation was revealed by the tests. Probably the unseparated flow pattern leads to the absence cavitation as well. To substantiate this conclusion, the experiments in a water tunnel are required.

Innovative Aspect and Main Advantages

The main innovative aspects of the new shapes are:

  • only special shaping was used in to prevent separation (in comparison with boundary-layer suction or other boundary-layer control methods ). Therefore, there is no additional energy supply to avoid separation. According to the Dalambert paradox, all new shapes have zero pressure drag at higher Reynolds numbers (when the boundary-layer thickness can be neglected).
  • the unseparated flow pattern could prevent cavitation, therefore such shapes could be used for vehicles, moving in water, to improve their cavitation characteristics.

All of these innovative points lead to a system, moving in a gas or liquid, that can supply the drag and noise diminishing and improving the cavitation inception characteristics.

Areas of Application

High-speed sub- and supersonic movement in liquid or in gas. Optimal design of hulls, winds, hydrofoils for radial pumps and compressors, other parts of the vehicles.


Fig. 1 Unseparated axisymmetric shape U-1, theoretical and experimental pressure distributions

Stage of Development

  • Axisymmetric models with different length of the positive pressure gradient (from 0.3% till 12% of the total body length) are available. They can be also used for the water tunnel tests to investigate the cavitation inception characteristics and to prove the existence of the unseparated flow pattern at higher Reynolds numbers.
  • Special 2D airfoils have to be manufactured and tested, to investigate their separation behavior at different angles of attack and to measure the drag and lift forces and the cavitation inception characteristics.
  • The next stages could be the theoretical and experimental investigations of supersonic shapes with negative pressure gradients over the surface, similar to shown in Pic.1, and presented hydrofoils in the radial pumps.

Contact Details

Prof. Igor Nesteruk
Institute of Hydromechanics of the National Academy of Sciences of Ukraine
E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it

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