The Laboratory for Hydraulic Machines says do not square the trailing edge. They wrote a paper about this for the hydro power industry. This is what Sailwors uses to reduce the whistle on AFS stabs. I can't attach it since it is a pdf file, so I will try a plain text paste here. Problem is that the paste omits the tables and drawings. However, I attached one of the drawings. #10 was the best design.
Chapter 1. Problem
overview
11
-Concepts of fluid-dynamic attenuation/control of vortex shedding
The control of the vortex shedding to reduce the vibration is presented through the following
methods:
a)Trailing edge geometry modification
Trailing edge geometry modification in the case of the elongated bodies such as vanes, blades,
and plates reduces vortex-induced excitation considerably. The effect of the trail- ing edge shape
on the vortex-induced vibration of plates has been investigated by different authors such as
Donaldson [17], Heskestad [37], Toebes [72] and Blake [13]. The summary of these studies is
illustrated in the Table 1.1.
Table 1.1: Trailing edge shapes investigated and their relative amplitude of vibration, Blake
[13]
Donaldson [17] performed systematic measurements of flow-induced vibration in Francisturbine
runners having different trailing edge shapes, Figure 1.8.
It was found that none of the trailing edge shape modifications changed the frequency of the
exciting forces. The vibration amplitude of different trailing edges versus velocity and also the
percentage amplitude in relation to the blunt trailing edge is presented in
EPFL - Laboratory for Hydraulic Machines
FAULTY TRANSITION PIECES
PREVIOUS HAND SANDING
NEW CNC CLIPPING JOB
12 I.
Introduction
Figure 1.8: Trailing edge shape, Donaldson [17]
Figure 1.9. Minimum vibration in comparison to the blunt trailing edge was found for the
trailing edge numbers 8, 9, 10.
The vortex shedding from a cambered hydrofoil with beveled trailing edge with angles of 44?
and 56? at high Reynolds number was studied by Dwayne [21]. He noted that the turbulent
fluctuations in the near wake depend on the Reynolds number due to the Re- dependence of
separating boundary layers on both sides of the hydrofoil, interacting to form the near wake. In
addition, the trailing edge geometry has an effect on the relative strength of the vortex shedding.
The ticker or blunter trailing edge produces stronger vortex shedding. Moreover, the
boundary layer is separated at the beveled side of the trailing edge and asymmetric vortex
shedding is found. Mosallem [52] investigated the characteristics of the flow past beveled
trailing edges attached to the flat plates with angles of 27? and 60?. The results display an
asymmetric wake behind the 27? beveled trailing edge contrary to the 60?case where a symmetric
wake is observed. Greenway [29] studied the effect of beveled trailing edge on vortex shedding
and vibration using a wind tunnel. It was found that the beveled trailing edge suppressed the vortex
excited vibration due to the rapid decay of the vortex resulting from an asymmetric circulation
distribution in the vortex formation region. He performed the measurement for different trailing
edge angles between 20? and 90?. As the trailing edge angle decreases, the vortex strength
becomes smaller. In addition, the vortices shed from the suction side are stronger than those from
the pressure side.
EPFL - Laboratory for Hydraulic
Machines
OPTIMUM TRAILING EDGE
The main challenge is that 1.8T
shall correspond to an approximate
value of <2mm
Chapter 1. Problem
overview
13
Figure 1.9: Vibration amplitude of various trailing edge shapes, Donaldson [17]
In the domain of hydraulic machine, fatigue cracks in stay vanes have occurred in
several hydro power stations over the years, [33], [48], and [27]. Two basic principles are
known to solve the problem, Grein [30]. The first is to modify the trailing edge to reduce the
vibration excitation and the second is to alter the natural frequency of the stay vanes. A
comprehensive review of stay vane cracking in hydraulic turbine has been presented by Gummer
[33]. The stay vane failure in the NKula Falls hydro power plant was resolved by modification
of trailing edge as reported by Fisher [24]. Lockey [49] noted the vortex-induced vibration
reduction in Ontario power generation by modifying the original trailing edge of stay vane to a Vshaped
trailing edge. The same modification was also reported for Little Long Generation station
north of Toronto by Goldwag [27].
EPFL - Laboratory for Hydraulic
Machines
