Flow pattern terminology

The terminology for flow patterns is a little confused at the present time, with different authors using different terms for the same pattern or using the same term with more or less precision. As it is commonly a matter of energetic debate and misunderstanding, the following comments are intended to clarify the terminology used here. They are written in the context of the figure given in Eurocode 1 Part 4 (see Fig. 1.3), followed by a fuller description of the terms used throughout this Guide.

“Mass flow” was clearly defined long ago as involving all the solid in a state of motion when the outlet is opened. It is helpful to reserve “mass flow” for this condition alone (see Fig. 1.4a).

“Expanded flow” silos (see Fig. 1.4d) present some terminological difficulties, as the lower hopper must be designed as a “mass flow hopper”, but the higher part involves stationary material. It is better not to refer to this container as a mass flow silo. The ASAE standard helpfully distinguishes between terms for hopper flow types and terms for silo flow patterns.

The term “funnel flow” was widely used by Jenike and later authors [e.g. Jenike, 1961; Jenike et al., 1973; Arnold et al., 1980; Roberts, 1998] for all flow patterns which are not mass flow. The need for a distinction between internal flows (see Fig. 1.4c) and flows with an effective transition against the silo wall (see Fig. 1.4b) was less clear in the 1970s than it is now, and because it has proved difficult to find a reliable criterion that can predict whether an effective transition will occur, authors have naturally been discouraged from making the distinction.

The French standard [AFNOR P22–630, 1992] and the ASAE standard [ASAE, 1989] restrict the term “funnel flow” to patterns with no effective transition. The DIN, ISO, Australian and Eurocode standards have adopted the Jenike view that all non-mass flows are funnel flows, and this accords with the widely used Jenike diagram (see Fig. 1.5) which indicates hopper

Page 16

slopes and wall frictional conditions required to distinguish mass flow from other flows, which are termed “funnel flow”. It therefore seems right to use “funnel flow” to mean all flows which are not mass flow, in line with Eurocode 1 [ENV 1991–4, 1995].

The term “core flow” is used in the BMHB draft code [BMHB, 1987] and Safarian and Harris [1985] and the German standard [DIN1055, 1987] “Kernfluβ’’ with the same meaning as Jenike’s wider use of “funnel flow”, though Nedderman [1992] restricts “core flow” to flows involving an effective transition. The term “funnel flow” is adopted here because of its apparently wider acceptance, but with an equivalence to “core flow” to match the previous BMHB advice.

A distinction is, however, needed between steep sided flow channels without an effective transition (see Fig. 1.4c) and wider channels which intersect the wall causing an effective transition (see Fig. 1.4b).

Flows that involve steep channels are termed “internal flow” by the draft Eurocode 1 and ISO standards. These standards do not cover highly eccentric outlets, so they do not consider steep sided flows which are in contact with the wall and are therefore not internal (a common feature of eccentric discharge). Thus, “internal flow” is misleading for these eccentric channel geometries, and a different term is desirable. Here, the term “internal flow” is reserved for truly internal flows. The French and ASAE standards use the term “funnel flow” for an internal flow, which must be rejected for the reasons given above. Other candidate terms are “pipe flow” and “chimney flow” (by adaptation of the French “cheminée” meaning a ship’s funnel). The term “pipe flow” is adopted here, though its apparent connection to the term “piping” used by Jenike [1961] and in the BMHB draft code [1987] to mean a stable empty channel or rathole should be avoided.

Where the pipe flow is internal, the flow has little effect on the wall pressures, and is here termed an “internal pipe flow”. Where it is in contact with the wall, it has a serious effect in locally reducing pressures and is here termed “eccentric pipe flow” (see Fig. 6.4) (Chapters 6 and 9). The “pipe” flows are separated into the

categories of “parallel pipe flow” and “taper pipe flow” to identify uniquely those common cases in which the channel has sides which are very close to vertical for the greater part of the silo (see Figs 6.3–6.5).

The condition of flow involving an effective transition (Fig. 1.4b) is variously known as “plug flow” [ASAE, 1989] “semi-mass flow” [Munch Andersen and Nielsen, 1986], “core flow” [Nedderman, 1992] or “mixed flow” [AFNOR P22–630, 1992]. The term “semi-mass flow” arises from the substantial segment of mass flowing material in the upper part of the silo, but it leads to confusion because of the reference to mass flow, so it is not used here. The term “core flow” usually has a wider meaning (see above), so it is inappropriate. The term “plug flow” is used by Drescher [1991] to mean

Page 17

an internal pipe flow with steep sides, which adds to the confusion. It probably originally referred only to the upper region of the flow zone in which mass flow is occurring (the translation of plug into French “tampon” provides a little challenge of explanation). The simpler term “mixed flow” appears to give a clearer direct meaning to a mixture of mass flow at the top and pipe flow at the bottom and so it is adopted here, and is recommended as a clear unique descriptor for flow patterns with an effective transition.

The terms adopted here are defined in Fig. 1.4. A more detailed description is given in Chapter 6 (see Figs 6.3– 6.5).